How odd! If you just need an email box, then just about any will do. If you are hosting one at the office, then you get into calendaring issues.
I'd pick the cheapest pop3/smtp capable one.
I have a host that offers three email programs !
1. Neo Mail
3. Squirrel Mail
I have no idea what server provides the best service and features.
Can someone who's used these servers before - point me in the right direction !