Chris Bartram already mentioned NetMail/3000 (and I have to second each
and every word he said, not because I have commercial interest in the
product but because I have used the full-blown trial version, including
the HP Desk gateway DeskLink/3000, and am still using the free 2-user
version of NetMail/3000)...

Another option might be Mark Bixby's recent "Sendmail/iX" port, which I
guess is available at http://www.cccd.edu/~markb or a few links below.

If you're not too familiar (or happy) with Posix-like tools and/or config
methods (and sendmail.cf does not seem an entry-level task to me), then
you might prefer the Netmail/3000 product because it is much more MPE-like
(read: VPLUS tool for configuring, server job for running the "daemons",
terminal interface cooperating with your terminal emulator in regard to
attachment up/download, etc).

Lars.