Hmm,
SBS 2003 comes with Exchange 2003. For it to have Exchange 2007 and WSS3 on it is, well, extremely unusual. Theoretically possible, but why would you upgrade just the Exchange component to 2007. You would be paying a fortune in licences when SBS 2008 does it "properly" for you.
I got muddled up about the email. Sharepoint 3, I believe, normally only sends email out using a simple SMTP service... which cannot coexist with an Exchange 2003 server. So outbound emails cannot work. This is documented in an MS Whitepaper on how to make WSS3 work on SBS2003.
But somehow on SBS 2008, WSS3 can do emails, whilst Exchange 2007 is present. I thought that whatever trick had been done here might be possible to add back in to the SBS 2003.
Anyway this is very techy for late on a Friday.... have a good weekend.
Alex