I have a problem with mail flow between two Exchange 2010 servers in separate AD sites.
Queues build up in both directions with error
451 4.4.0 Primary target IP address responded with: "421 4.2.1 Unable to connect." Attempted failover to alternate host, but that did not succeed. Either there are not alternate hosts, or delivery failed to alternate hosts.
Delivery type of the queue is SMTP Relay to Remote Active Directory Site.
Browsing to the EWS site (or even the root of the site) of one server from the other via HTTPS fails. HTTP works correctly.
The problem is limited to these two servers. Everything works fine from other Exchange servers in the organisation, and browsing to the EWS site from other hosts within either of the two affected sites works without a problem.
So the problem appears to be down to HTTPS communication between these two Exchange servers in particular. All certificates are valid and bindings appear to be correct.
Any thoughts?