Qmail-smtpd

From Qmail Info Wiki

qmail-smtpd is the part of qmail which handles the server end of an SMTP connection.

Normally, a program like tcpserver will actually listen to the appropriate port, and when it detects a connection attempt which isn't blocked by its own access rules, it accepts the socket and runs qmail-smtpd to handle the SMTP conversation. On some servers it may run rblsmtpd first, in order to reject connections from IP addresses which are listed in an RBL, or "realtime blackhole list".

You can read more about qmail-smtpd by typing "man qmail-smtpd" on a qmail sytem. If it does not show you the man page, you may need to add /var/qmail/man to your /etc/man.conf file.