How do I resolve the following on our exchange server when some end users in our org. try to reply back to certain recipients?

#554 5.4.4 SMTPSEND.DNS.NonExistentDomain; nonexistent domain ##
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Alan HardistyCo-OwnerCommented:
I would imagine that the FQDN on your send connector is invalid / reporting an internal domain name that ends in .local or similar and the receiving server is checking the FQDN and it is coming back as unresolveable.
Check the FQDN on your Send Connector and make sure it is resolveable in DNS.  It should be something like and that FQDN should resolve back to your sending IP Address.

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For testing purpose add the IP address and FQDN of exchange in host file and check mail flow.
If this works fine then you need to check the settings in your internal DNS server
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