sendmail. It provides a way to allow legitimate users to relay mail through an
SMTP server, while still preventing others from using it as a spam relay.
User's IP addresses are added to the map immediately after they have
authenticated to the POP or IMAP server. By default, map entries expire after
30 minutes, but can be renewed by additional authentication. Periodically
checking mail on a POP server is sufficient to do this. The POP and SMTP
servers can be on different hosts.
