I have SBS 2003 server than can receive external emails, except when sent from a SBS 2008 or SBS 2010 server. If I try a smtpdiag from the 2008/2010 servers, it shows as succesful. No information on any of the logs. The 2010/2008 Queue will just show the connection as active. Nothing on the incoming logs on the 2003 server.
I called Microsoft and they said that it was a dns issue with the ISP, but I have other SBS 2003 servers with the same ISP and they have no problems receiving mail from 2010/2008 servers.