Hmm, a 600mhz, Win98 box used as a printer server and internt gateway at the moment
lol, now we have a problem.
The only mail server I would suggest is Merak, or something. Qmail isnt bad either.
These may not run very well on win 98.
Win 98 as an internet gateway is a bit poor, personally, either upgrade to Windows server 2003 SMB edition, which is £155 for OEM version I think, for 5 clients.
Thats oneway, or get Linux. Linux will be mroe secure / faster / reliable and will do everything your w98 box does. :-) Plus it can do mail server too, becuse you have postfix, and I'm sure you could find some way of making it work with MS outlook with server data access.
However, please be warned that Linux can be complex to set-up - but it would be best if you could get someone in who knows about linux. :-)