I have an aged SBS2003 box which is now, not sending e-mail. Receive yes, send no. SMTP seems to be working as I can telnet to it and get a response. The server has mail sent to, and sent from, directly, no connectors. I can look-up remote server MX records with NSLookup and I can browse the web without issues. According to MXTools.com, the server is not on any blacklist.
It just stopped working. I am not sure why.
I have; rebooted the server, restarted all of the services, modified the registry to allow for Exchange databases >18GB in size (current db size is 18.1GB), tried a new forwarding DNS server.