Have you checked the settings as per the MS article? :
http://support.microsoft.com/kb/937635
Might also be worth running the SBS 2003 Best Practise Analyzer and Exchange 2003 Best Practise Analyzer:
http://www.microsoft.com/downloads/details.aspx?familyid=3874527A-DE19-49BB-800F-352F3B6F2922&displaylang=en
http://www.microsoft.com/downloads/details.aspx?FamilyID=DBAB201F-4BEE-4943-AC22-E2DDBD258DF3&displaylang=en