1. Mail proxy for firewalls with anti-spam and anti-relay features 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. From Debian 3.0r0 APT