postfix-master/OVERVIEW.html

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   8 <title>Postfix Architecture Overview </title>
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  15
  16 <h1> <img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix
  17 Architecture Overview </h1>
  18
  19 <hr>
  20
  21 <h2> Introduction </h2>
  22
  23 <p> This document presents an overview of the Postfix architecture,
  24 and provides pointers to descriptions of every Postfix command
  25 or server program.  The text gives the general context in which
  26 each command or server program is used, and provides pointers to
  27 documents with specific usage examples and background information.
  28 </p>
  29
  30 <p> Topics covered by this document: </p>
  31
  32 <ul>
  33
  34 <li> <a href="#receiving"> How Postfix receives mail </a>
  35
  36 <li> <a href="#delivering"> How Postfix delivers mail </a>
  37
  38 <li> <a href="#behind"> Postfix behind the scenes </a>
  39
  40 <li> <a href="#commands"> Postfix support commands </a>
  41
  42 </ul>
  43
  44 <h2><a name="receiving"> How Postfix receives mail </a> </h2>
  45
  46 <p> When a message enters the Postfix mail system, the first stop
  47 on the inside is the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a>. The figure below shows the
  48 main processes that are involved with new mail.  Names followed by
  49 a number are Postfix commands or server programs, while unnumbered
  50 names inside shaded areas represent Postfix queues. </p>
  51
  52 <blockquote>
  53
  54 <table>
  55
  56 <tr>
  57
  58 <td colspan="4"> </td>
  59
  60 <td bgcolor="#f0f0ff" align="center"> <a href="trivial-rewrite.8.html">trivial-<br>rewrite(8)</a> </td>
  61
  62 </tr>
  63
  64 <tr>
  65
  66 <td> Network </td> <td> <tt> -&gt; </tt> </td>
  67
  68 <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="smtpd.8.html">smtpd(8)</a>
  69 </td>
  70
  71 <td> </td>
  72
  73 <td rowspan="2" align="center"> <table> <tr> <td align="center">
  74 ^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt>
  75 </td> </tr> </table> </td>
  76
  77 </tr>
  78
  79 <tr>
  80
  81 <td colspan="3"> </td> <td> <tt> \ </tt> </td>
  82
  83 </tr>
  84
  85 <tr>
  86
  87 <td> Network </td> <td> <tt> -&gt; </tt> </td>
  88
  89 <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="qmqpd.8.html">qmqpd(8)</a>
  90 </td>
  91
  92 <td> <tt> -&gt; </tt> </td>
  93
  94 <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="cleanup.8.html">cleanup(8)</a>
  95 </td>
  96
  97 <td> <tt> -&gt; </tt> </td>
  98
  99 <td bgcolor="#f0f0ff" align="center" valign="middle"> <a
 100 href="QSHAPE_README.html#incoming_queue"> incoming </a> </td>
 101
 102 </tr>
 103
 104 <tr>
 105
 106 <td colspan="3"> </td> <td> <tt> / </tt> </td>
 107
 108 </tr>
 109
 110 <tr>
 111
 112 <td colspan="2"> </td>
 113
 114 <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="pickup.8.html">pickup(8)</a>
 115 </td>
 116
 117 <td> <tt> &lt;- </tt> </td>
 118
 119 <td bgcolor="#f0f0ff" align="center" valign="middle"> <a
 120 href="QSHAPE_README.html#maildrop_queue"> maildrop </a> </td>
 121
 122 </tr>
 123
 124 <tr>
 125
 126 <td colspan="4" align="center"> </td>
 127
 128 <td align="center"> ^<br> <tt> | </tt> </td>
 129
 130 </tr>
 131
 132 <tr>
 133
 134 <td> Local </td> <td> <tt> -&gt; </tt> </td>
 135
 136 <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="sendmail.1.html">sendmail(1)</a>
 137 </td>
 138
 139 <td> <tt> -&gt; </tt> </td>
 140
 141 <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="postdrop.1.html">postdrop(1)</a>
 142 </td>
 143
 144 </tr>
 145
 146 </table>
 147
 148 </blockquote>
 149
 150 <ul>
 151
 152 <li> <p> Network mail enters Postfix via the <a href="smtpd.8.html">smtpd(8)</a> or <a href="qmqpd.8.html">qmqpd(8)</a>
 153 servers.  These servers remove the SMTP or QMQP protocol encapsulation,
 154 enforce some sanity checks to protect Postfix, and give the sender,
 155 recipients and message content to the <a href="cleanup.8.html">cleanup(8)</a> server.  The
 156 <a href="smtpd.8.html">smtpd(8)</a> server can be configured to block unwanted mail, as
 157 described in the <a href="SMTPD_ACCESS_README.html">SMTPD_ACCESS_README</a> document.  </p>
 158
 159 <li> <p> Local submissions are received with the Postfix <a href="sendmail.1.html">sendmail(1)</a>
 160 compatibility command, and are queued in the <a href="QSHAPE_README.html#maildrop_queue">maildrop queue</a> by
 161 the privileged <a href="postdrop.1.html">postdrop(1)</a> command. This arrangement even works
 162 while the Postfix mail system is not running.  The local <a href="pickup.8.html">pickup(8)</a>
 163 server picks up local submissions, enforces some sanity checks to
 164 protect Postfix, and gives the sender, recipients and message
 165 content to the <a href="cleanup.8.html">cleanup(8)</a> server.  </p>
 166
 167 <li> <p> Mail from internal sources is given directly to the
 168 <a href="cleanup.8.html">cleanup(8)</a> server. These sources are not shown in the figure, and
 169 include: mail that is forwarded by the <a href="local.8.html">local(8)</a> delivery agent (see
 170 next section), messages that are returned to the sender by the
 171 <a href="bounce.8.html">bounce(8)</a> server (see second-next section), and postmaster
 172 notifications about problems with Postfix.  </p>
 173
 174 <li> <p> The <a href="cleanup.8.html">cleanup(8)</a> server implements the final processing
 175 stage before mail is queued. It adds missing From: and other message
 176 headers, and transforms addresses as described in the
 177 <a href="ADDRESS_REWRITING_README.html">ADDRESS_REWRITING_README</a>
 178 document. Optionally, the <a href="cleanup.8.html">cleanup(8)</a> server can be configured to
 179 do light-weight content inspection with regular expressions as
 180 described in the <a href="BUILTIN_FILTER_README.html">BUILTIN_FILTER_README</a> document.  The  <a href="cleanup.8.html">cleanup(8)</a>
 181 server places the result as a single file into the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a>,
 182 and notifies the queue manager (see next section) of the arrival
 183 of new mail.  </p>
 184
 185 <li> <p> The <a href="trivial-rewrite.8.html">trivial-rewrite(8)</a> server rewrites addresses to the
 186 standard "user@fully.qualified.domain" form, as described in the
 187 <a href="ADDRESS_REWRITING_README.html">ADDRESS_REWRITING_README</a> document. Postfix currently does not
 188 implement a rewriting language, but a lot can be done via table
 189 lookups and, if need be, regular expressions. </p>
 190
 191 </ul>
 192
 193 <h2> <a name="delivering"> How Postfix delivers mail </a> </h2>
 194
 195 <p> Once a message has reached the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a> the next step is
 196 to deliver it. The figure shows the main components of the Postfix
 197 mail delivery apparatus. Names followed by a number are Postfix
 198 commands or server programs, while unnumbered names inside shaded
 199 areas represent Postfix queues. </p>
 200
 201 <blockquote>
 202
 203 <table>
 204
 205 <tr>
 206
 207 <td rowspan="2" colspan="4"> </td>
 208
 209 <td rowspan="2" bgcolor="#f0f0ff" align="center"> <a href="trivial-rewrite.8.html">trivial-<br>rewrite(8)</a>
 210 </td>
 211
 212 <td> </td>
 213
 214 <td bgcolor="#f0f0ff" align="center"> <a href="smtp.8.html">smtp(8)</a> </td>
 215
 216 <td> <tt> -&gt; </tt> </td> <td> Network </td>
 217
 218 </tr>
 219
 220 <tr>
 221
 222 <td align="right"> <tt> / </tt> </td>
 223
 224 </tr>
 225
 226 <tr>
 227
 228 <td rowspan="2" colspan="4"> </td>
 229
 230 <td rowspan="2" align="center"> <table> <tr> <td align="center">
 231 ^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt>
 232 </td> </tr> </table> </td>
 233
 234 <td align="right"> <tt> - </tt> </td>
 235
 236 <td bgcolor="#f0f0ff" align="center"> <a href="lmtp.8.html">lmtp(8)</a> </td>
 237
 238 <td> <tt> -&gt; </tt> </td> <td> Network </td>
 239
 240 </tr>
 241
 242 <tr>
 243
 244 <td align="left"> <tt> / </tt> </td>
 245
 246 </tr>
 247
 248 <tr>
 249
 250 <td bgcolor="#f0f0ff" align="center"> <a
 251 href="QSHAPE_README.html#incoming_queue"> incoming </a> </td>
 252
 253 <td> <tt> -&gt; </tt> </td>
 254
 255 <td bgcolor="#f0f0ff" align="center"> <a
 256 href="QSHAPE_README.html#active_queue"> active </a> </td>
 257
 258 <td> <tt> -&gt; </tt> </td>
 259
 260 <td bgcolor="#f0f0ff" align="center"> <a href="qmgr.8.html">qmgr(8)</a> </td>
 261
 262 <td align="right"> <tt> --- </tt> </td>
 263
 264 <td bgcolor="#f0f0ff" align="center"> <a href="local.8.html">local(8)</a> </td>
 265
 266 <td> <tt> -&gt; </tt> </td> <td> File, command </td>
 267
 268 </tr>
 269
 270 <tr>
 271
 272 <td rowspan="2" colspan="2"> </td>
 273
 274 <td rowspan="2" align="center"> <table> <tr> <td align="center">
 275 ^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt>
 276 </td> </tr> </table> </td>
 277
 278 <td rowspan="2" colspan="2"> </td>
 279
 280 <td align="left"> <tt> \ </tt> </td>
 281
 282 </tr>
 283
 284 <tr>
 285
 286 <td align="right"> <tt> - </tt> </td>
 287
 288 <td bgcolor="#f0f0ff" align="center"> <a href="virtual.8.html">virtual(8)</a> </td>
 289
 290 <td> <tt> -&gt; </tt> </td> <td> File </td>
 291
 292 </tr>
 293
 294 <tr>
 295
 296 <td colspan="2"> </td>
 297
 298 <td bgcolor="#f0f0ff" align="center"> <a
 299 href="QSHAPE_README.html#deferred_queue"> deferred </a> </td>
 300
 301 <td colspan="2"> </td>
 302
 303 <td align="right"> <tt> \ </tt> </td>
 304
 305 </tr>
 306
 307 <tr>
 308
 309 <td colspan="6">
 310
 311 <td bgcolor="#f0f0ff" align="center"> <a href="pipe.8.html">pipe(8)</a> </td>
 312
 313 <td> <tt> -&gt; </tt> </td> <td> Command </td>
 314
 315 </tr>
 316
 317 </table>
 318
 319 </blockquote>
 320
 321 <ul>
 322
 323 <li> <p> The queue manager (the <a href="qmgr.8.html">qmgr(8)</a> server process in the
 324 figure) is the heart of Postfix mail delivery.  It contacts the
 325 <a href="smtp.8.html">smtp(8)</a>, <a href="lmtp.8.html">lmtp(8)</a>, <a href="local.8.html">local(8)</a>, <a href="virtual.8.html">virtual(8)</a>, <a href="pipe.8.html">pipe(8)</a>, <a href="discard.8.html">discard(8)</a> or
 326 <a href="error.8.html">error(8)</a> delivery agents, and sends a delivery request for one
 327 or more recipient addresses. The <a href="discard.8.html">discard(8)</a> and <a href="error.8.html">error(8)</a> delivery
 328 agents are special: they discard or bounce all mail, and are not
 329 shown in the figure above.  </p>
 330
 331 <p> The queue manager maintains a small <a href="QSHAPE_README.html#active_queue">active queue</a> with the
 332 messages that it has opened for delivery. The <a href="QSHAPE_README.html#active_queue">active queue</a> acts as
 333 a limited window on potentially large <a href="QSHAPE_README.html#incoming_queue">incoming</a> or <a href="QSHAPE_README.html#deferred_queue">deferred queues</a>.
 334 The limited <a href="QSHAPE_README.html#active_queue">active queue</a> prevents the queue manager from running
 335 out of memory under heavy load. </p>
 336
 337 <p> The queue manager maintains a separate <a href="QSHAPE_README.html#deferred_queue">deferred queue</a> for mail
 338 that cannot be delivered, so that a large mail backlog will not
 339 slow down normal queue accesses. The queue manager's strategy for
 340 delayed mail delivery attempts is described in the <a href="QSHAPE_README.html">QSHAPE_README</a>
 341 and <a href="TUNING_README.html">TUNING_README</a> documents. </p>
 342
 343 <li> <p> The <a href="trivial-rewrite.8.html">trivial-rewrite(8)</a> server resolves each recipient
 344 address according to its local or remote address class, as defined
 345 in the <a href="ADDRESS_CLASS_README.html">ADDRESS_CLASS_README</a> document.  Additional routing information
 346 can be specified with the optional <a href="transport.5.html">transport(5)</a> table.  The
 347 <a href="trivial-rewrite.8.html">trivial-rewrite(8)</a> server optionally queries the <a href="relocated.5.html">relocated(5)</a> table
 348 for recipients whose address has changed; mail for such recipients is
 349 returned to the sender with an explanation. </p>
 350
 351 <li> <p> The <a href="smtp.8.html">smtp(8)</a> client looks up a list of mail exchangers for
 352 the destination host, sorts the list by preference, and tries each
 353 server in turn until it finds a server that responds. It then
 354 encapsulates the sender, recipient and message content as required
 355 by the SMTP protocol; this includes conversion of 8-bit MIME to
 356 7-bit encoding. </p>
 357
 358 <li> <p> The <a href="lmtp.8.html">lmtp(8)</a> client speaks a protocol similar to SMTP that
 359 is optimized for delivery to mailbox servers such as Cyrus.  The
 360 advantage of this setup is that one Postfix machine can feed multiple
 361 mailbox servers over LMTP. The opposite is true as well:  one
 362 mailbox server can be fed over LMTP by multiple Postfix machines.
 363 </p>
 364
 365 <li> <p> The <a href="local.8.html">local(8)</a> delivery agent understands UNIX-style mailboxes,
 366 qmail-compatible maildir files, Sendmail-style system-wide <a href="aliases.5.html">aliases(5)</a>
 367 databases, and Sendmail-style per-user .forward files. Multiple
 368 local delivery agents can be run in parallel, but parallel delivery
 369 to the same user is usually limited. </p>
 370
 371 <p> The <a href="local.8.html">local(8)</a> delivery agent has hooks for alternative forms of
 372 local delivery: you can configure it to deliver to mailbox files
 373 in user home directories, you can configure it to delegate mailbox
 374 delivery to an external command such as procmail, or you can delegate
 375 delivery to a different Postfix delivery agent.  </p>
 376
 377 <li> <p> The <a href="virtual.8.html">virtual(8)</a> delivery agent is a bare-bones delivery
 378 agent that delivers to UNIX-style mailbox or qmail-style maildir
 379 files only.  This delivery agent can deliver mail for multiple
 380 domains, which makes it especially suitable for hosting lots of
 381 small domains on a single machine.  This is described in the
 382 <a href="VIRTUAL_README.html">VIRTUAL_README</a> document. </p>
 383
 384 <li> <p> The <a href="pipe.8.html">pipe(8)</a> mailer is the outbound interface to other mail
 385 processing systems (the Postfix <a href="sendmail.1.html">sendmail(1)</a> command being the
 386 inbound interface).  The interface is UNIX compatible: it provides
 387 information on the command line and on the standard input stream,
 388 and expects a process exit status code as defined in &lt;sysexits.h&gt;.
 389 Examples of delivery via the <a href="pipe.8.html">pipe(8)</a> mailer are in the <a href="MAILDROP_README.html">MAILDROP_README</a>
 390 and <a href="UUCP_README.html">UUCP_README</a> documents.
 391
 392 </ul>
 393
 394 <h2> <a name="behind"> Postfix behind the scenes </a> </h2>
 395
 396 <p> The previous sections gave an overview of how Postfix server
 397 processes send and receive mail. These server processes rely on
 398 other server processes that do things behind the scenes.  The text
 399 below attempts to visualize each service in its own context.  As
 400 before, names followed by a number are Postfix commands or server
 401 programs, while unnumbered names inside shaded areas represent
 402 Postfix queues. </p>
 403
 404 <ul>
 405
 406 <li> <p> The resident <a href="master.8.html">master(8)</a> server is the supervisor that keeps
 407 an eye on the well-being of the Postfix mail system. It is typically
 408 started at system boot time with the "postfix start" command, and
 409 keeps running until the system goes down.  The <a href="master.8.html">master(8)</a> server is
 410 responsible for starting Postfix server processes to receive and
 411 deliver mail, and for restarting servers that terminate prematurely
 412 because of some problem. The <a href="master.8.html">master(8)</a> server is also responsible
 413 for enforcing the server process count limits as specified in the
 414 <a href="master.5.html"><b>master.cf</b></a> configuration file. The picture below gives the
 415 program hierarchy when Postfix is started up. Only some of the mail
 416 handling daemon processes are shown. </p>
 417
 418 <table>
 419
 420 <tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff">
 421 <a href="postfix.1.html">postfix(1)</a> </td> </tr>
 422
 423 <tr> <td colspan="2"> </td> <td align="center"> |<br> |</td> </tr>
 424
 425 <tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff">
 426 postfix-script(1) </td> </tr>
 427
 428 <tr> <td> </td> <td> <table> <tr> <td> </td> <td> / </td> </tr>
 429 <tr> <td> / </td> <td> </td> </tr> </table> </td> <td align="center">
 430 |<br> |</td> <td> <table> <tr> <td> \ </td> <td> </td> </tr> <tr>
 431 <td> </td> <td> \ </td> </tr> </table> </td> </tr>
 432
 433 <tr> <td align="center" bgcolor="#f0f0ff"> <a href="postsuper.1.html">postsuper(1)</a> </td> <td>
 434 </td> <td align="center" bgcolor="#f0f0ff"> <a href="master.8.html">master(8)</a> </td> <td>
 435 </td> <td align="center" bgcolor="#f0f0ff"> <a href="postlog.1.html">postlog(1)</a> </td> </tr>
 436
 437 <tr> <td> </td> <td> <table> <tr> <td> </td> <td> / </td> </tr>
 438 <tr> <td> / </td> <td> </td> </tr> </table> </td> <td align="center">
 439 |<br> |</td> <td> <table> <tr> <td> \ </td> <td> </td> </tr> <tr>
 440 <td> </td> <td> \ </td> </tr> </table> </td> </tr>
 441
 442 <tr> <td align="center" bgcolor="#f0f0ff"> <a href="smtpd.8.html">smtpd(8)</a> </td> <td>
 443 </td> <td align="center" bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a> </td> <td>
 444 </td> <td align="center" bgcolor="#f0f0ff"> <a href="local.8.html">local(8)</a> </td> </tr>
 445
 446 </table>
 447
 448 <li> <p> The <a href="anvil.8.html">anvil(8)</a> server implements client connection and
 449 request rate
 450 limiting for all <a href="smtpd.8.html">smtpd(8)</a> servers.  The <a href="TUNING_README.html">TUNING_README</a> document
 451 provides guidance for dealing with mis-behaving SMTP clients. The
 452 <a href="anvil.8.html">anvil(8)</a> service is available in Postfix version 2.2 and later.
 453 </p>
 454
 455 <table>
 456
 457 <tr> <td> Network </td> <td> <tt> -&gt; </tt> </td> <td align="center"
 458 bgcolor="#f0f0ff"> <br> <a href="smtpd.8.html">smtpd(8)</a><br><br> </td> <td> <tt> &lt;-&gt;
 459 </tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="anvil.8.html">anvil(8)</a><br><br>
 460 </td> </tr>
 461
 462 </table>
 463
 464 <li> <p> The <a href="bounce.8.html">bounce(8)</a>, <a href="defer.8.html">defer(8)</a> and <a href="trace.8.html">trace(8)</a> services each maintain
 465 their own queue directory trees with per-message logfiles. Postfix
 466 uses this information when sending "failed", "delayed" or "success"
 467 delivery status notifications to the sender. </p>
 468
 469 <p> The <a href="trace.8.html">trace(8)</a> service also implements support for the Postfix
 470 "sendmail
 471 -bv" and "sendmail -v" commands which produce reports about how
 472 Postfix delivers mail, and is available with Postfix version 2.1
 473 and later. See <a href="DEBUG_README.html#trace_mail"> DEBUG_README
 474 </a> for examples.  </p>
 475
 476 <table>
 477
 478 <tr> <td align="center" bgcolor="#f0f0ff"> <a href="cleanup.8.html">cleanup(8)</a> </td> <td
 479 valign="middle"> <tt> -&gt; </tt> </td> <td align="center"
 480 bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a><br> Postfix<br> queue </td> <td
 481 valign="middle"> <tt> -&gt; </tt> </td> <td align="center"
 482 bgcolor="#f0f0ff"> Delivery<br> agents</td> </tr>
 483
 484 <tr> <td align="center"> ^<br> <tt> | </tt> </td> <td> </td> <td
 485 align="center"> <tt> |<br> v </tt> </td> <td> </td> <td align="center">
 486 <tt> |<br> v </tt> </td> </tr>
 487
 488 <tr> <td align="center"> (Non-)<br> delivery<br> notice </td> <td
 489 valign="middle"> <tt> &lt;- </tt> </td> <td align="center"
 490 bgcolor="#f0f0ff"> <a href="bounce.8.html">bounce(8)</a><br> <a href="defer.8.html">defer(8)</a><br> <a href="trace.8.html">trace(8)</a> </td> <td
 491 valign="middle"> <tt> &lt;- </tt> </td> <td align="center"> Queue
 492 id,<br> recipient,<br> status</td> </tr>
 493
 494 <tr> <td colspan="2"> </td> <td align="center"> <table> <tr> <td
 495 align="center"> ^<br> <tt> | </tt> </td> <td align="center"> <tt>
 496 |<br> v </tt> </td> </tr> </table> </td> </tr>
 497
 498 <tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff">
 499 Per- <br> message<br> logfiles </td> </tr>
 500
 501 </table>
 502
 503 <li> <p> The <a href="flush.8.html">flush(8)</a> servers maintain per-destination logs and
 504 implement both ETRN and "sendmail -qRdestination", as described
 505 in the <a href="ETRN_README.html">ETRN_README</a> document. This moves selected queue files from
 506 the <a href="QSHAPE_README.html#deferred_queue">deferred queue</a> back to the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a> and requests their
 507 delivery.  The <a href="flush.8.html">flush(8)</a> service is available with Postfix version
 508 1.0 and later.  </p>
 509
 510 <table>
 511
 512 <tr> <td colspan="4"> </td> <td align="center" bgcolor="#f0f0ff">
 513 <a href="QSHAPE_README.html#incoming_queue"> incoming </a><br>^
 514 <br><a href="QSHAPE_README.html#deferred_queue"> deferred </a>
 515 </td> </tr>
 516
 517 <tr> <td colspan="4"> </td> <td align="center"> ^<br> |</td> </tr>
 518
 519 <tr> <td align="center" bgcolor="#f0f0ff"> <a href="smtpd.8.html">smtpd(8)</a><br> <a href="sendmail.1.html">sendmail(1)</a><br>
 520 <a href="postqueue.1.html">postqueue(1)</a> </td> <td> <tt> - </tt> </td> <td align="center">
 521 Destination<br> to flush</td> <td> <tt> -&gt; </tt> </td> <td
 522 align="center" bgcolor="#f0f0ff"> <a href="flush.8.html">flush(8)</a> </td> <td> <tt> &lt;-
 523 </tt> </td> <td align="center"> Deferred<br> destination,<br> queue
 524 id </td> <td> <tt> - </tt> </td> <td align="center" bgcolor="#f0f0ff">
 525 Delivery<br> agents,<br> <a href="qmgr.8.html">qmgr(8)</a> </td> </tr>
 526
 527 <tr> <td colspan="4"> </td> <td align="center"> <table> <tr> <td
 528 align="center"> ^<br> <tt> | </tt> </td> <td align="center"> <tt>
 529 |<br> v </tt>  </td> </tr> </table> </td> </tr>
 530
 531 <tr> <td colspan="4"> </td> <td align="center"> Per-dest-<br>
 532 ination<br> logs </td> </tr>
 533
 534 </table>
 535
 536 <li> <p> The <a href="proxymap.8.html">proxymap(8)</a> servers provide read-only and read-write
 537 table lookup
 538 service to Postfix processes. This overcomes chroot restrictions,
 539 reduces the number of open lookup tables by sharing one open
 540 table among multiple processes, and implements single-updater
 541 tables. </p>
 542
 543 <li> <p> The <a href="scache.8.html">scache(8)</a> server maintains the connection cache for
 544 the Postfix <a href="smtp.8.html">smtp(8)</a> client. When connection caching is enabled for
 545 selected destinations, the <a href="smtp.8.html">smtp(8)</a> client does not disconnect
 546 immediately after a mail transaction, but gives the connection to
 547 the connection cache server which keeps the connection open for a
 548 limited amount of time.  The <a href="smtp.8.html">smtp(8)</a> client continues with some
 549 other mail delivery request. Meanwhile, any <a href="smtp.8.html">smtp(8)</a> process can
 550 ask the <a href="scache.8.html">scache(8)</a> server for that cached connection and reuse it
 551 for mail delivery. As a safety measure, Postfix limits the number
 552 of times that a connection may be reused.  </p>
 553
 554 <p> When delivering mail to a destination with multiple mail servers,
 555 connection caching can help to skip over a non-responding server,
 556 and thus dramatically speed up delivery. SMTP connection caching
 557 is available in Postfix version 2.2 and later.  More information
 558 about this feature is in the <a href="CONNECTION_CACHE_README.html">CONNECTION_CACHE_README</a> document. </p>
 559
 560 <table>
 561
 562 <tr> <td> </td> <td> <tt> /-- </tt> </td> <td align="center"
 563 colspan="3" bgcolor="#f0f0ff"> <a href="smtp.8.html">smtp(8)</a> </td> <td colspan="2"> <tt>
 564 --&gt; </tt> Internet </td> </tr>
 565
 566 <tr> <td align="center" bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a> </td> <td> </td>
 567 <td align="center" rowspan="3"> &nbsp; </td> <td align="center"
 568 rowspan="3"><tt>|<br>|<br>|<br>|<br>v</tt></td> <td> &nbsp; </td>
 569 </tr>
 570
 571 <tr> <td> </td> <td> <tt> \-- </tt> </td> <td align="center"
 572 colspan="2" bgcolor="#f0f0ff"> <a href="smtp.8.html">smtp(8)</a> </td> <td align="left"> <tt>
 573 --&gt; </tt> Internet </td> </tr>
 574
 575 <tr> <td colspan="3"> </td> <td align="center"><tt>^<br>|</tt></td>
 576 <td> &nbsp; </td> </tr>
 577
 578 <tr> <td colspan="3"> </td> <td align="center" colspan="3"
 579 bgcolor="#f0f0ff"> <a href="scache.8.html">scache(8)</a> </td> </tr>
 580
 581 </table>
 582
 583 <li> <p> The <a href="showq.8.html">showq(8)</a> servers list the Postfix queue status. This
 584 is the queue listing service that does the work for the <a href="mailq.1.html">mailq(1)</a>
 585 and <a href="postqueue.1.html">postqueue(1)</a> commands.  </p>
 586
 587 <table>
 588
 589 <tr> <td> Output </td> <td> <tt> &lt;- </tt> </td> <td align="center"
 590 bgcolor="#f0f0ff"> <a href="mailq.1.html">mailq(1)</a><br>
 591
 592 <a href="postqueue.1.html"> post-<br>queue(1) </a> <br> </td> <td>
 593 <tt> &lt;- </tt> </td> <td align="center" valign="middle"
 594 bgcolor="#f0f0ff"> <a href="showq.8.html">showq(8)</a> </td> <td> <tt> &lt;- </tt></td> <td
 595 align="center" valign="middle" bgcolor="#f0f0ff"> Postfix<br> queue
 596 </td> </tr>
 597
 598 </table>
 599
 600 <li> <p> The <a href="spawn.8.html">spawn(8)</a> servers run non-Postfix commands on request,
 601 with the client connected via socket or FIFO to the command's
 602 standard input, output and error streams. You can find examples of
 603 its use in the <a href="SMTPD_POLICY_README.html">SMTPD_POLICY_README</a> document.  </p>
 604
 605 <li> <p> The <a href="tlsmgr.8.html">tlsmgr(8)</a> server runs when TLS (Transport Layer
 606 Security, formerly known as SSL) is turned on in the Postfix <a href="smtp.8.html">smtp(8)</a>
 607 client or <a href="smtpd.8.html">smtpd(8)</a> server. This process has two duties: </p>
 608
 609 <ul>
 610
 611 <li> <p> Maintain the pseudo-random number generator (PRNG) that
 612 is used to seed the TLS engines in Postfix <a href="smtp.8.html">smtp(8)</a> client or <a href="smtpd.8.html">smtpd(8)</a>
 613 server processes.  The state of this PRNG is periodically saved to
 614 a file, and is read when <a href="tlsmgr.8.html">tlsmgr(8)</a> starts up. </p>
 615
 616 <li> <p> Maintain the optional Postfix <a href="smtp.8.html">smtp(8)</a> client or <a href="smtpd.8.html">smtpd(8)</a>
 617 server caches with TLS session keys. Saved keys can improve
 618 performance by reducing the amount of computation at the start of
 619 a TLS session. </p>
 620
 621 </ul>
 622
 623 <p> TLS support is available in Postfix version 2.2 and later.
 624 Information about the Postfix TLS implementation is in the <a href="TLS_README.html">TLS_README</a>
 625 document. </p>
 626
 627 <table>
 628
 629 <tr> <td>Network<tt>-&gt; </tt> </td> <td align="center"
 630 bgcolor="#f0f0ff"> <br> <a href="smtpd.8.html">smtpd(8)</a> <br> &nbsp; </td> <td colspan="2">
 631 <tt> &lt;---seed---<br><br>&lt;-session-&gt; </tt> </td> <td
 632 align="center" bgcolor="#f0f0ff"> <br> <a href="tlsmgr.8.html">tlsmgr(8)</a> <br> &nbsp; </td>
 633 <td colspan="3"> <tt> ---seed---&gt;<br> <br>&lt;-session-&gt;
 634 </tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="smtp.8.html">smtp(8)</a> <br>
 635 &nbsp; </td> <td> <tt> -&gt;</tt>Network </td> </tr>
 636
 637 <tr> <td colspan="3"> </td> <td align="right"> <table> <tr> <td>
 638 </td> <td> / </td> </tr> <tr> <td> / </td> <td> </td> </tr> </table>
 639 </td> <td align="center"> |<br> |</td> <td align="left"> <table>
 640 <tr> <td> \ </td> <td> </td> </tr> <tr> <td> </td> <td> \ </td>
 641 </tr> </table> </td> <td colspan="3"> </td> </tr>
 642
 643 <tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff">
 644 smtpd<br> session<br> cache </td> <td> </td> <td align="center"
 645 bgcolor="#f0f0ff"> PRNG<br> state <br>file </td> <td> </td> <td
 646 align="center" bgcolor="#f0f0ff"> smtp<br> session<br> cache </td>
 647 <td colspan="2"> </td> </tr>
 648
 649 </table>
 650
 651
 652 <li> <p> The <a href="verify.8.html">verify(8)</a> server verifies that a sender or recipient
 653 address is deliverable before the <a href="smtpd.8.html">smtpd(8)</a> server accepts it.  The
 654 <a href="verify.8.html">verify(8)</a> server queries a cache with address verification results.
 655 If a result is not found, the <a href="verify.8.html">verify(8)</a> server injects a probe
 656 message into the Postfix queue and processes the status update from
 657 a delivery agent or queue manager.
 658 This process is described in the <a href="ADDRESS_VERIFICATION_README.html">ADDRESS_VERIFICATION_README</a>
 659 document.  The <a href="verify.8.html">verify(8)</a> service is available with Postfix version
 660 2.1 and later. </p>
 661
 662 <table>
 663
 664 <tr>
 665
 666     <td rowspan="2" colspan="5" align="center" valign="middle">
 667     &nbsp; </td> <td rowspan="3" align="center" valign="bottom">
 668     <tt> -&gt; </tt> </td> <td rowspan="3" align="center"
 669     valign="middle"> probe<br> message </td> <td rowspan="3"
 670     align="center" valign="middle"> <tt> -&gt; </tt> </td> <td
 671     rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle">
 672     Postfix<br> mail<br> queue </td>
 673
 674 </tr>
 675
 676 <tr> <td> </td> </tr>
 677
 678 <tr>
 679
 680     <td rowspan="3" align="center" valign="middle"> Network </td>
 681     <td rowspan="3" align="center" valign="middle"> <tt> -&gt; </tt>
 682     </td> <td rowspan="3" bgcolor="#f0f0ff" align="center"
 683     valign="middle"> <a href="smtpd.8.html">smtpd(8)</a> </td> <td rowspan="3" align="center"
 684     valign="middle"> <tt> &lt;-&gt; </tt> </td> <td rowspan="3"
 685     bgcolor="#f0f0ff" align="center" valign="middle"> <a href="verify.8.html">verify(8)</a>
 686     </td>
 687
 688 </tr>
 689
 690 <tr>
 691
 692     <td rowspan="1" colspan="3"> </td> <td rowspan="1" align="center"
 693     valign="middle"> <tt> |</tt><br> <tt> v</tt> </td>
 694
 695 </tr>
 696
 697 <tr>
 698
 699     <td rowspan="3" align="center" valign="top"> <tt> &lt;- </tt>
 700     </td> <td rowspan="3" align="center" valign="middle"> probe<br>
 701     status </td> <td rowspan="3" align="center" valign="middle">
 702     <tt> &lt;- </tt> </td> <td rowspan="3" bgcolor="#f0f0ff"
 703     align="center" valign="middle"> Postfix<br> delivery<br> agents
 704     </td> <td rowspan="3" align="left" valign="middle"> <tt>-&gt;</tt>
 705     Local<br> <tt>-&gt;</tt> Network</td>
 706
 707 </tr>
 708
 709 <tr>
 710
 711     <td rowspan="3" colspan="4" align="center" valign="middle">
 712     &nbsp; </td> <td rowspan="3" align="center" valign="middle">
 713     <tt> ^</tt><br> <tt> |</tt><br> <tt> v</tt> </td>
 714
 715 </tr>
 716
 717 <tr> <td> </td> </tr>
 718
 719 <tr> <td colspan="4"> &nbsp; </td> </tr>
 720
 721 <tr>
 722
 723     <td colspan="4" align="center" valign="middle"> &nbsp; </td>
 724     <td bgcolor="#f0f0ff" align="center" valign="middle"> Address<br>
 725     verification<br> cache </td>
 726
 727 </tr>
 728
 729 </table>
 730
 731 <li> <p> The <a href="postscreen.8.html">postscreen(8)</a> server can be put "in front" of Postfix
 732 <a href="smtpd.8.html">smtpd(8)</a> processes. Its purpose is to accept connections from the
 733 network and to decide what SMTP clients are allowed to talk to
 734 Postfix.  According to the 2008 MessageLabs annual report, 81% of
 735 all email was spam, and 90% of that was sent by botnets; by 2010,
 736 those numbers were 92% and 95%, respectively.  While <a href="postscreen.8.html">postscreen(8)</a>
 737 keeps the zombies away, more <a href="smtpd.8.html">smtpd(8)</a> processes remain available
 738 for legitimate clients.  </p>
 739
 740 <p> The <a href="postscreen.8.html">postscreen(8)</a> server is available with Postfix 2.8 and
 741 later. To keep the implementation simple, <a href="postscreen.8.html">postscreen(8)</a> delegates
 742 DNS white/backlist lookups to <a href="dnsblog.8.html">dnsblog(8)</a> server processes, and
 743 delegates TLS encryption/decryption to <a href="tlsproxy.8.html">tlsproxy(8)</a> server processes.
 744 This delegation is invisible to the remote SMTP client, and is not
 745 shown in the diagram below.  </p>
 746
 747 <table>
 748
 749 <tr> <td> zombie </td> </tr>
 750
 751 <tr> <td>  </td> <td align="left"> <tt> \ </tt> </td> </tr>
 752
 753 <tr> <td> zombie </td> <td align="left"> <tt> - </tt> </td> <td>
 754 </td> <td>  </td> <td>  </td> <td align="right"> <tt> - </tt> </td>
 755 <td bgcolor="#f0f0ff" align="center"> <a href="smtpd.8.html">smtpd(8)</a> </td> </tr>
 756
 757 <tr> <td>  </td> <td align="right"> <tt> \ </tt> </td> <td> </td>
 758 <td align="left"> <tt> / </tt> </td> </tr>
 759
 760 <tr> <td bgcolor="#f0f0ff" align="center"> other </td> <td> <tt>
 761 --- </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle">
 762 <a href="postscreen.8.html">postscreen(8)</a> </td> </tr>
 763
 764 <tr> <td>  </td> <td align="right"> <tt> / </tt> </td> <td> </td>
 765 <td align="right"> <tt> \ </tt> </td> </tr>
 766
 767 <tr> <td bgcolor="#f0f0ff" align="center"> other </td> <td align="left">
 768 <tt> - </tt> </td> <td> </td> <td>  </td> <td>  </td> <td align="right">
 769 <tt> - </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="smtpd.8.html">smtpd(8)</a>
 770 </td> </tr>
 771
 772 <tr> <td>  </td> <td align="left"> <tt> / </tt> </td> </tr>
 773
 774 <tr> <td> zombie </td> </tr>
 775
 776
 777 </table>
 778
 779 </ul>
 780
 781 <h2> <a name="commands"> Postfix support commands </a> </h2>
 782
 783 <p> The Postfix architecture overview ends with a summary of
 784 command-line utilities for day-to-day use of the Postfix mail
 785 system. Besides the Sendmail-compatible <a href="sendmail.1.html">sendmail(1)</a>, <a href="mailq.1.html">mailq(1)</a>, and
 786 <a href="newaliases.1.html">newaliases(1)</a> commands, the Postfix system comes with it own
 787 collection of command-line utilities.  For consistency, these are
 788 all named post<i>something</i>. </p>
 789
 790 <ul>
 791
 792 <li> <p> The <a href="postfix.1.html">postfix(1)</a> command controls the operation of the mail
 793 system. It is the interface for starting, stopping, and restarting
 794 the mail system, as well as for some other administrative operations.
 795 This command is reserved to the super-user. </p>
 796
 797 <li> <p> The <a href="postalias.1.html">postalias(1)</a> command maintains Postfix <a href="aliases.5.html">aliases(5)</a> type
 798 databases.  This is the program that does the work for the
 799 <a href="newaliases.1.html">newaliases(1)</a> command.  </p>
 800
 801 <li> <p> The <a href="postcat.1.html">postcat(1)</a> command displays the contents of Postfix
 802 queue files. This is a limited, preliminary utility. This program
 803 is likely to be superseded by something more powerful that can also
 804 edit Postfix queue files. </p>
 805
 806 <li> <p> The <a href="postconf.1.html">postconf(1)</a> command displays or updates Postfix <a href="postconf.5.html">main.cf</a>
 807 parameters and displays system dependent information about the
 808 supported file locking methods, and the supported types of lookup
 809 tables.  </p>
 810
 811 <li> <p> The <a href="postdrop.1.html">postdrop(1)</a> command is the mail posting utility that
 812 is run by the Postfix <a href="sendmail.1.html">sendmail(1)</a> command in order to deposit mail
 813 into the <a href="QSHAPE_README.html#maildrop_queue">maildrop queue</a> directory. </p>
 814
 815 <li> <p> The <a href="postkick.1.html">postkick(1)</a> command makes some Postfix internal
 816 communication channels available for use in, for example, shell
 817 scripts. </p>
 818
 819 <li> <p> The <a href="postlock.1.html">postlock(1)</a> command provides Postfix-compatible mailbox
 820 locking for use in, for example, shell scripts. </p>
 821
 822 <li> <p> The <a href="postlog.1.html">postlog(1)</a> command provides Postfix-compatible logging
 823 for shell scripts. </p>
 824
 825 <li> <p> The <a href="postmap.1.html">postmap(1)</a> command maintains Postfix lookup tables
 826 such as <a href="canonical.5.html">canonical(5)</a>, <a href="virtual.5.html">virtual(5)</a> and others. It is a cousin of the
 827 UNIX makemap command. </p>
 828
 829 <li> <p> The <a href="postmulti.1.html">postmulti(1)</a> command repeats the "postfix start" etc.
 830 command for each Postfix instance, and supports creation, deletion
 831 etc. of Postfix instances. For a tutorial, see <a href="MULTI_INSTANCE_README.html">MULTI_INSTANCE_README</a>.
 832 </p>
 833
 834 <li> <p> The <a href="postqueue.1.html">postqueue(1)</a> command is the privileged command that
 835 is run by Postfix <a href="sendmail.1.html">sendmail(1)</a> and <a href="mailq.1.html">mailq(1)</a> in order to flush or
 836 list the
 837 mail queue. </p>
 838
 839 <li> <p> The <a href="postsuper.1.html">postsuper(1)</a> command maintains the Postfix queue. It
 840 removes old temporary files, and moves queue files into the right
 841 directory after a change in the hashing depth of queue directories.
 842 This command is run at mail system startup time and when Postfix
 843 is restarted. </p>
 844
 845 </ul>
 846
 847 </body>
 848
 849 </html>