I think I found the cause – awaiting some test results to confirm.

 

 

 

Try an NSLOOKUP type=MX on the recipient @host.  That can be very telling. 

You may need to telnet port 25 to each of them and begin a simulated RFC821 dialog.  Maybe a VRFY will fail.

In my former career, I've seen misconfigured MX hosts that would only be a problem when the preferred host wasn't available.

 

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