I had one server running Exchange Server 2003sp2 on WinServer2003sp2, sending mail successfully to the Internet. Then I installed Exchange Server 2007sp1 on WinServer2008x64. In EMC, I created an "additional" Send Connector, along with the Exchange 2003 connector that was already there. After, creating this additional Send Connector, mailboxes on each server cannot send mail to the Internet. If you open ESM, the messages are held in the "Messages with an unreachable destination," queue. Once the Exchange 2007 server is turned off the messages are sent to the Internet fine for the Exchange 2003 Server.
The below article seems to be related to my problem, however I need some detailed steps on how this can be accomplished if this is the recommended solution:
"When you introduce Exchange Server 2007 into an existing organization, in order to route mail you absolutely must have routing group connectors and an SMTP connector.
You'll need two routing group connectors, one going from the Exchange Server 2007 routing group to the Exchange Server 2003 routing group, and vice versa. You can set this up as part of the installation process, but if you missed it or you're not sure, you can check using the Exchange Management Shell and correct the problem there. If you don't, you will not be able to send mail between your servers. Messages will end up in unreachable destination queues."