It's easy to build a mail server on linux, maybe a little too easy.
It takes a bit of tweeking and planning to keep spammers out of inboxes and large messages (pictures and video) can quickly bring your server to its knee's.
Google's Apps For Your Domain is a great free service that great for small buisnesses of 50-200 Employees
Google Apps would be an interesting choice for anyone who isn't already committed to a technology. To replace the calendaring functions of Outlook one might consider Calgoo as a partial replacement. Examples of Linux Email Servers include Sendmail and Scalix.
David.
Isn't that usually a problem with IMAP implementations across the board?