postfix-master/receiving.html

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   5 <title>Postfix Anatomy - Receiving Mail</title>
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  12 <h1><a href="big-picture.html"><img src="small-picture.gif" width="115" height="45"></a> Postfix
  13 Anatomy - Receiving Mail</h1>
  14
  15 <hr>
  16
  17 <p> <strong> Note: this web page is no longer maintained. It exists
  18 only to avoid breaking links in web pages that describe earlier
  19 versions of the Postfix mail system. </strong> </p>
  20
  21 <a href="docs.html">Up one level</a> | Receiving Mail | <a
  22 href="delivering.html">Delivering Mail</a> | <a
  23 href="backstage.html">Behind the Scenes</a> | <a
  24 href="commands.html">Command-line Utilities</a>
  25
  26 <p>
  27
  28 When a message enters the Postfix mail system, the first stop on
  29 the inside is the <b>incoming</b> queue.  The figure below shows
  30 the main components that are involved with new mail. For an
  31 explanation of the symbols used, click on the icon in the upper
  32 left-hand corner of this page.
  33
  34 <p>
  35
  36 <center>
  37
  38 <img src="inbound.gif" width="497" height="213">
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  40 </center>
  41
  42 <p>
  43
  44 <ul>
  45
  46 <li>Mail is posted locally. The Postfix <a
  47 href="sendmail.1.html">sendmail</a> program invokes the privileged
  48 <a href="postdrop.1.html">postdrop</a> program which deposits the
  49 message into the <b>maildrop</b> directory, where the message is
  50 picked up by the <a href="pickup.8.html">pickup</a> daemon.  This
  51 daemon does some sanity checks, in order to protect the rest of
  52 the Postfix system.
  53
  54 <p>
  55
  56 <li>Mail comes in via the network.  The Postfix <a href="smtpd.8.html">SMTP
  57 server</a> receives the message and does some sanity checks, in
  58 order to protect the rest of the Postfix system.  The SMTP server
  59 can be configured to implement <a href="uce.html">UCE</a> controls
  60 on the basis of local or network-based black lists, DNS lookups,
  61 and other client request information.
  62
  63 <p>
  64
  65 <li>Mail is generated internally by the Postfix system itself, in
  66 order to return undeliverable mail to the sender.  The <a
  67 href="bounce.8.html">bounce or defer</a> daemon brings the bad
  68 news.
  69
  70 <p>
  71
  72 <li>Mail is forwarded by the <a href="local.8.html">local</a>
  73 delivery agent, either via an entry in the system-wide <a
  74 href="aliases.5.html">alias</a> database, or via an entry in a
  75 per-user <a href="aliases.5.html">.forward</a> file. This is
  76 indicated with the unlabeled arrow.
  77
  78 <p>
  79
  80 <li>Mail is generated internally by the Postfix system itself, in
  81 order to notify the postmaster of a problem (this path is also
  82 indicated with the unlabeled arrow). The Postfix system can be
  83 configured to <a href="basic.html#notify">notify</a> the postmaster
  84 of SMTP protocol problems, UCE policy violations, and so on.
  85
  86 <p>
  87
  88 <li>The <a href="cleanup.8.html">cleanup</a> daemon implements the
  89 final processing stage for new mail. It adds missing <b>From:</b>
  90 and other message headers, arranges for address rewriting to the
  91 standard <i>user@fully.qualified.domain</i> form, and optionally
  92 extracts recipient addresses from message headers. The <b> cleanup</b>
  93 daemon inserts the result as a single queue file into the
  94 <b>incoming</b> queue, and notifies the <a href="qmgr.8.html">queue
  95 manager</a> of the arrival of new mail.  The <b>cleanup</b> daemon
  96 can be configured to transform addresses on the basis of <a
  97 href="rewrite.html#canonical">canonical</a> and <a
  98 href="rewrite.html#virtual">virtual</a> table lookups.
  99
 100 <p>
 101
 102 <li>On request by the <b>cleanup</b> daemon, the <a
 103 href="trivial-rewrite.8.html">trivial-rewrite</a> daemon rewrites
 104 addresses to the standard <i>user@fully.qualified.domain</i> form.
 105 The initial Postfix version does not implement a rewriting language.
 106 Implementing one would take a lot of effort, and most sites do not
 107 need it.  Instead, Postfix makes extensive use of <a
 108 href="rewrite.html">table lookup</a>.
 109
 110 </ul>
 111
 112 <hr>
 113
 114 <a href="docs.html">Up one level</a> | Receiving Mail | <a
 115 href="delivering.html">Delivering Mail</a> | <a
 116 href="backstage.html">Behind the Scenes</a> | <a
 117 href="commands.html">Command-line Utilities</a>
 118
 119 </body>
 120
 121 </html>