While working on a migration involving Exchange Server 2003 recently we encountered an interesting bug with the SMTP connectors in Exchange Server 2003 which is not publicly documented by Microsoft.

The diagram below demonstrates the configuration in Exchange Server 2003 that we were using

The configuration that we had in place was where the Exchange 2003 bridgehead server hosted multiple SMTP connectors, some of which were for specific routes but also included a connector for the default route.

What we were finding was that when an email was sent to a @source.local address we would get a 5.4.4 No Route found NDR report returned from the Boundary SMTP Smart host. As you can see from the above diagram the emails for @source.local should never be routed to the Boundary device. Using WinRoute we could see that the routing table on the bridgehead server hosting the connectors was correct and contained the entry for the specific route.

After a support call with Microsoft we discovered that there is a bug in Exchange Server 2003 with the way that the Categorizer handles SMTP connectors when there is an internet based connector present. I had not seen this during any previous migrations but it manifested itself for this client.

The resolution in this particular case was to move the more specific connector up onto Exchange Server 2007 as this particular environment had Exchange 2007 deployed so we used this architecture.

If you are not fortunate enough to have Exchange 2007 or Exchange 2010 running in conjunction with Exchange 2003 the only real alternative if this problem occurs is to deploy another Exchange 2003 server to act as a bridgehead for the more specific route and remove the specific route from the bridgehead server hosting the Internet based connector using an architecture similar to that shown below.