The procmail mail processing program can be used to create mail-servers,
mailing lists, sort your incoming mail into separate folders/files (real
convenient when subscribing to one or more mailing lists or for prioritising
your mail), preprocess your mail, start any programs upon mail arrival
(e.g. to generate different chimes on your workstation for different
types of mail) or selectively forward certain incoming mail automatically
to someone.

Procmail can be used:
        - and installed by an unprivileged user (for himself only).
        - as a drop in replacement for the local delivery agent /bin/mail
          (with biff/comsat support).
        - as a general mailfilter for whole groups of messages (e.g. when
          called from within sendmail.cf rules).

The accompanying formail program enables you to generate autoreplies,
split up digests/mailboxes into the original messages, do some very
simple header-munging/extraction, or force mail into mail-format (with
leading From line).