Section mail - Debian Package: smtpd

 Smtpd uses two programs, smtpd which listens for incoming mail and places
 it in a private queue, and smtpfwdd, which invokes sendmail to deliver
 messages from the queue. The configuration file for smtpd has powerful
 features for controlling the acceptance of incoming mail.
 .
 It is particularly useful for firewall sites, where the policy aspects of
 which mail should be accepted (in smtpd's configuration file) are clearly
 separated from the routing and other aspects of the mail configuration
 (which remain in the sendmail configuration file)
 .
 It can be used with any mail-transport-agent which can be invoked as sendmail
 but is intended to integrate with the Debian sendmail package, which
 recognises when smtpd has been installed.
 .
 If you are attempting to use it with an MTA other than sendmail then you
 will need to be prepared to modify the MTA configuration.