I have 3 exchange servers. 2 in London(NLB) and 1 in Singapore. they are on same domain connected via wan. active directory sites and services seem to be setup ok.
we have the domain bsxxx.com mx record pointing to london. emails arrive fine to users who have a mailbox in london. If a user in singapore who's mailbox is on the singapore exchange server has an email address with this domain, sometimes the emails flow through fine, and sometime the queue builds up.
When i look at the queue viewer on the london server it shows
SMTP Relay to Remote Active Directory Site
451 4.4.0 Primary target IP address responded with: "421 4.3.2 Service not available, closing transmission channel." Attempted failover to alternative host, but that did not succeed.
If i reboot the Singapore server, the queues will deliver when the server is backup. But this only lasts for an hour or so.
also there is a local domain bsxxx.com.sg and that points to the Singapore server, and that works fine to local Singapore mailboxes.