Mail.app and Microsoft Outlook 2011 both only support Exchange 2007 and later. What mail clients for Snow Leopard support Exchange 2003? I'm especially interested in email. Being able to view calendar events would be a bonus.
You can look at davmail. It is an Exchange gateway and will allow you to read-write the Exchange calendar from iCal, or to use any email application with Exchange.
Note that if your administrator allows IMAP access to your Exchange account, you do not need client support specifically for Exchange.