Hi Mike
Preferably it should run on windows (we're just not at home on the *nix platforms), have all it's config options in a sql database, provide anti-spam and anti-virus out of the box, have a feature-rich webmail client and be tailored for a small ISP.
Our specs: ~700 Domains, ~4000 Users
MS exchange? CERN uses it: http://mmmservices.web.cern.ch/mmmservices/Help/?kbid=211030 http://mmm.cern.ch
The current version seems quite stable. But I'm just a user, I don't know how it is to manage.
cheers Edoardo