I have an SBS 2003 server running Exchange. Periodically, when sending to a remote domain it fails with an NDR, usually no such recipient. Looking in the SMTP logs I've noted that when the emails go through successfully, the Exchange server is sending to the correct IP address. When the emails fails, the Exchange server is trying to send to a wrong address, which happens to be the same domains WWW record IP address, which happens to be running an SMTP server, but kicks it back as no such recipeint. The remote domain is running their own mail server, not hosting it.