smtpd is a lightweight mail server whose sole
purpose is to receive incoming messages and
deliver them to local users. Its main "feature"
(or non-feature, really) is to be completely
unable to relay messages to adjacent servers: if
an incoming message is not addressed to a local
mailbox, smtpd will reject it. Another convenient
feature is its automatic grey-listing of clients
that allows you to efficiently reject spammers
based on their behaviour. It can handle incoming
email traffic for an unlimited number of domains
(i.e. act as a MX for these domains) without the
need for any configuration (other than creation of
users' mailboxes).
