Recently while performing the initial steps for an Exchange 2003 to 2007 migration, I ran into one of the more frustrating issues with Exchange 2003 to 2007 transitions. Broken inter-organization mail flow. After banging my head against the wall for a few hours, and sending enough test emails to get added to who knows how many OBLs, I finally stumbled on the answer.
Exchange 2007 could talk to the 2003 environment as well as route mail outbound to the Internet, however all 03 to 07 mail was getting jammed in the queues. The customer was running Exchange 2003 in an Active-Passive cluster configuration and had problems previously with their Default SMTP Virtual Server. Their resolution was to disable it and create a new SMTP VS Instance as a cluster resource.
When you install Exchange 2007, by default it creates two routing group connectors and binds them to the SMTP VS instance with the index of 1 on the Exchange 2003 side. In this case the disabled Default VS Instance. Luckily this client had a front end 03 server with it’s own Default VS Instance. I was able to restore mail flow by removing the Routing Group Connectors that 07 put in by default using Power Shell, then create new ones that attached to the front end server rather than the mailbox cluster. This also allowed for me to list multiple Hub Transport servers in my connector rather than just the first one installed in the organization. By listing multiple HT servers, redundancy is achieved during the migration period.
To remove the connectors:
How to Remove Routing Group Connectors in Exchange 2007
To create the new ones:
How to Create Routing Group Connectors from Exchange 2007 to Exchange Server 2003