This is another _really_ minimal and naive ISO C90-ish smtp server for linux:
 - single-threaded (1 connection = 1 set of mail transactions)
 - source configured (look for the configuration section in the code)
 - epoll-ed
 - signalfd-ed (i.e. synchronous)
 - direct syscalls (no libc)
 - maildir format mailboxes (no lock)

Will chroot in the configured path once started, usually /var/mail where
maildir format mailboxes should have been created by you.

Don't forget you need to give root priviledges to the server in order to listen
on port 25.

Look in ulinux/patterns/network_server if you want to ramp up this server with
cloning, etc. Carefull, SIGCHLD is not sufficient to account properly for dead or
exited children, even with signalfd, because only the latest killed/exited
child is reported.