Mail flows one way from one 2007 box, but not vice versa

Hi,

I currently have two exchange servers in two sites. The plan is to move all mailboxes from Site A to Site B so we can decomission the server, but this is a project that will take a few months to complete so both servers need to send mail back and forth depending on where users mailboxes are.

Site A is old site, Site B is the new.

Site A has User A in its mailbox DB
Site B has User B in its mailbox DB

User A can send mail to User B, external can send to User B
BUT
User B cannot send to anyone at Site A, but can to others at Site B

The messages sit in the queue and report "4.4.0 Primary target IP address responded with: “454.4.7.0 Temporary authentication failure.”  Attempted failover to alternate host, but that did not succeed.  Either there are no alternate hosts or deliver failed to all alternate hosts. "

In the eventlogs the Transport Service throws Error 2017 "Outbound authentication failed with error TargetUnknown for Send connector Intra-Organization SMTP Send Connector. The authentication mechanism is ExchangeAuth. The target is SMTPSVC/externalFQDN.domain.com"

externalFQDN.domain.com is what users use for connecting to OWA etc so has been changed for security.

What I dont understand is why it appears to want to go the external route, and not through the internal route to server.domain.local at Site A.

Any help that anyone can suggest would be greatly appreciated as this is holding up a project.

CHeers,
RIch.