As part of a migration to Exchange 2013, I've added a second Exchange 2007 server to my domain. After installation-- during which I installed Mailbox, Client Access, and Hub transport roles--I moved some test mailboxes to the new server. The test mailboxes can send/receive from other mailboxes on the old server, and can RECEIVE from outside the domain, but cannot SEND emails to outside addresses.
After ~24 hours I receive a delivery delay, and finally an Undeliverable error from the second server with a 4.4.7 QUEUE.Expired message.
My understanding was that the two Exchange servers would detect the AD set-up and work together, so I didn't make any changes to any send/receive connectors, and they appear identical. I also haven't done anything with external DNS or NAT, as I understand everything flows through the original server.
First Exch2007 server is SP1 RU10 (I can't remove this and install SP3, which is why I decided to build a second Exchange server). Server OS is Server 2003 SP2. VM
Second Exch2007 server is SP3. Server OS is Server 2008 R2 SP1. VM