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Exchange Error 421 4.2.1 Unable to Connect

Last Modified: 2016-09-10
I have 2 Exchange 2010 servers (EX01 and EX02) in a domain (Domain A). Hub Transport is installed on both and the DBs are in a DAG. Recently, we upgraded an Exchange server in a different domain (Domain B - same physical LAN) from Exchange 2007 to Exchange 2010. We essentially copied settings from the 2007 server to the new 2010 server in regards to Hub Transport, Receive Connectors, etc. During this time, the new server's IP address was and the old server (2007) was After all settings were configured, we swapped IPs and verified that MX records updated in each domain's DNS. The new server (2010) is now .40 and the old one (2007) was shut off.

Shortly after, we noticed that emails from Domain A are delayed to Domain B. Eventually, they do get through however. The error is the Queue viewer is "Primary Target IP address responded with: 421 4.2.1 Unable to Connect. Attempted failover to alternate host, but that did not succeed."

After some Google searching, I found the logs and found this...
2016-03-07T16:36:23.154Z,08D33CD71D67B094,SMTP,domainB.com,>,Failed connection to (TimedOut:0000274C)[TargetHost:srvexch01-ed.domainB.com|MarkedUnhealthy|FailureCount:1|NextRetryTime:2016-03-07T16:37:23.154Z][TargetIPAddress:|MarkedUnhealthy|FailureCount:1|NextRetryTime:2016-03-07T16:37:23.154Z]
2016-03-07T16:36:23.154Z,08D33CD71D67B094,SMTP,domainB.com,-,Messages: 0 Bytes: 0 (Retry : Unable to connect)

I noticed that sometimes it is trying to connect to which was the IP of the new 2010 exchange server before we swapped IPs. If DNS shows the correct IP to name translation, how is this happening? Where can I clear that entry?

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