451 4.4.0 primary target IP address responded with "421.4.4.2 unable to connect."attempted failover to alternate host, but that did not succeed.Either there are no alternate hosts, or delivery failed

We are getting the error message in our queue for what looks like two domains but no others

 "451 4.4.0 primary target IP address responded with "421.4.4.2 unable to connect."attempted failover to alternate host, but that did not succeed.Either there are no alternate hosts, or delivery failed to all alternate hosts."

We have an Exchange 2010 server and we have not made any changes recently and email flow has been okay otherwise.  

The rest of our clients are okay. Is it on their end? Any assistance offered would be greatly appreciated.
regsampAsked:
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Amit KumarCommented:
Please provide if you get any NDR.

for troubleshooting please check destination domain's MX is getting resolved on this server and you can telnet this MX on 25 with EHLO packet.

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regsampAuthor Commented:
I am sorry for the delay. I had an personal emergency. I checked the destination MX and was not getting resolved and I could not telnet to them. It ended up being a mimecast issue. Thank you for your help.
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