We have three back-end Exchange 2003 servers, all SP2. There is no front-end server.
Two are in one site - we'll call those London1 and London2. The other is at a remote site, we'll call that Remote1.
Up until today, London2 and Remote1 have London1 configured as a Smarthost. This then send's e-mail using DNS lookup. Today we setup with a 3rd party mail filter, Messagelabs. I configured London1 to relay all e-mail via Messagelabs (I.e. I configured messagelabs as a smarthost under the smtp virtual server on London1).
Since adding the smarthost, some of the e-mails sent from mailboxes on London1 to mailboxes on the other servers end up in a queue called remote1.ourdomain.com and london2.ourdomain.com and then move quite quickly (couple of minutes) to the 'Messages queued for deferred delivery' queue.
If I take the smarthost out of London1, mail flows around the company fine.
I want to use the smarthost for all mail that leaves the organisation but I also want mail to flow properly between my back-end servers. I think the solution might lie in routing groups and connectors but I'm not really sure what I'm doing with those.