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 is designed to handle three common types of mail delivery
between a dialup computer and an ISP:

   * Delivery to the ISP. The dialup computer sends all outgoing mail to
     the ISP.

   * AutoTURN delivery from the ISP. After the dialup computer makes an
     SMTP connection to the ISP, it automatically receives an SMTP
     connection back from the ISP if there is any new mail for it. This
     provides the same power as ETRN but does not require a special
     client.

   * User-controlled delivery from the ISP. A user with a shell account
     can switch from qmail-pop3d to serialmail without pestering the
     sysadmin. The user can also decide whether undeliverable messages
     should be left for POP retrieval or bounced back to the sender.

serialmail supports SMTP, including ESMTP PIPELINING, and QMTP.

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