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 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
serialmail's maildir2smtp without pestering the sysadmin. The user can
also decide whether undeliverable messages should be left for POP
retrieval or bounced back to the sender.
