So I just finished diagnosing an Exchange 2003 replication issue. This Issue drove me crazy and took about 40 hours to track down. I read everything I could find, analyzed a million (felt like it) event viewer entries and IIS\SMTP logs.

Of course it was simple.

Exchange uses SMTP to send messages to other Exchange Servers in the Organization to keep replicated information update to date.

Well I was taking a client from a single Mail server to a more secure FE\BE (Frontend\Backend) setup.

After clearing all other issues. The new backend was still unable to send Replication messages to the old server via SMTP. Exchange System Manager (ESM) kept saying "Connection dropped by remote host" on the outgoing SMTP que.

Well as it turns out...The last IT service provider had an IP Address buried in the SMTP virtual server settings on the old server. This setting in essence said to only allow incoming SMTP connections from this particular address. An address that was also an unknown host....documentation would have been nice in this scenario.