This question gets asked quite a bit in the newsgroups. Here’s one consis response to this situation, with some alternatives, that was originally posted at APC’s web site: http://www.apcmag.com/apc/v3.nsf/0/C0038E31CA6AA56ACA256E750013E5FC
Instant messaging — where did it go?
The instant messaging server function in Exchange 2000 was deleted from Exchange 2003 and thus from SBS 2003. For large corporations this isn’t a big problem as they can keep an Exchange 2000 IM Server running in their Exchange 2003 environment; but what can be done on an SBS server?
One option is to use one of the free instant messaging services such as ICQ, MSN Messenger or Yahoo! Messenger. This does have some drawbacks though; extra traffic over the Internet connection can be costly. The added risk of malware entering the network also has to be taken into consideration. Most importantly, potentially sensitive company information will be sent over the Internet without any security. For some small businesses this could be an acceptable risk; for others it isn’t.
Then there’s Jabber, a familiar program to many IM users as it connects to several different IM networks. Jabber clients are free and the server is also free, at least if you run it on a Linux box.
There’s also a wealth of single purpose IM programs out there. Our tests of LeadMind Development’s Popmessenger gave good results.