Microsoft Mail Alternative???

All 50 users on our Win2K domain use Microsoft Mail for intraoffice emailing and only 30 of them have the Internet Email through a local ISP. As more and more client pc are upgraded to Window and Office XP, we start to run into issue where those who don’t have Internet Email cannot use Microsoft Mail since it is not supported under Outlook XP/2003. An alternative we had to ‘downgrade’ their Outlook2003 to OL2000.
We considered implementing Exchange 2000 to solve this issue but we’re lack of man power to maintain emails in-house.

Besides Exchange Server and Instant Messaging, is there any other emailing tools that are commonly used and safe for intra-office mails?

Any input is appreciated.