> rebuild the box (an old G3 desktop) with a fresh OS 9 and SIMS installs

Bad idea. SIMS' state-of-the-90s antispam options plus Mac OS 9's
state-of-the-90s underpinnings are no longer a winning combination, alas.

> replace the box with a new mac, OS X and communigate pro - or some
> open source postfix thingy

If you have 5 or fewer users, CGPro has a free community edition for you. If
you have more, then that option could get quite expensive very quickly. I
prefer Exim as an MTA, but it's worth checking out Mac OS X server, which at
$500 for the 10-user edition delivers a pretty solid
Postfix/Courier/Squirrelmail setup with a nice GUI front end. Be aware that
Mac OS X is a very different beast from Mac OD 9: If you already administer
Mac OS X boxen great; if not then allow for a learning curve.

> replace the box with a linux box, run fedora and postfix/sendmail on it

If you're an experienced Linux admin, this makes sense. If not, Mac OS X
will probably be easier to learn and administer.
--
Dave Pooser
Cat-Herder-in-Chief, Pooserville.com
"...Life is not a journey to the grave with the intention of arriving
safely in one pretty and well-preserved piece, but to slide across the
finish line broadside, thoroughly used up, worn out, leaking oil, and
shouting GERONIMO!!!" -- Bill McKenna