serialmail is a collection of tools for passing mail across serial
links. It works with qmail: you use qmail to deliver messages to a
maildir, and then serialmail to deliver messages out of the maildir.

serialmail uses ucspi-tcp/tcpclient for networking. It can also be used
with future UCSPI clients for transparent compression, IPv6, etc.

serialmail supports SMTP, including ESMTP PIPELINING, and QMTP.

serialmail is under user control. A user delivering messages to a
maildir, and picking up the messages through qmail-pop3d, can switch to
maildirsmtp without pestering the sysadmin. The user can also decide
whether undeliverable messages should be left for POP retrieval or
bounced back to the sender.

http://pobox.com/~djb/serialmail.html