Take a look at Chapter 17 of the SBS 2003 Unleashed book for more detail about what I'm sharing here. MacMail does NOT have true Exchange connectivity, even though one of the options for mail server is "Exchange." Mac Mail can only make POP3 or IMAP connections to mail servers, and the Exchange option in MacMail assumes an IMAP connection to the Exchange server.
By default, IMAP and POP3 are not configured in SBS 2003.
You have two options:
1. Enable the IMAP service in Exchange and configure MacMail to use that.
2. Get a copy of Entourage from Office 2004 or Office 2008 and connect to Exchange with that.
Unless the Mac user specifically does NOT want to get to his calendar or contact information in Exchange, I would strongly recommend using Entourage instead of MacMail.
Information about configuring Entourage 2004 to connect to SBS can be found in the SBS 2003 Unleashed book, but can also be found at http://simultaneouspancakes.com/Les...sbs-2003/,
which is actually a little more up to date than the information in the book.