Mail flow Stopped to External in Queue – SMTP Relay to Active Directory Site
Intermittent Emails are not sent to External World.
Queue Error – “451 4.4.0 primary target IP address responded with “451 5.7.3 Cannot achieve Exchange Server authentication.”
Solution –
Two Mailbox Servers in the Environment .
One of the Exchange Server IP was added to Relay Connectors of the other Exchange Server.
Eventually Breaking the Exchange Authentication . Causing Any email submitted to 2 mailbox server cannot transmit/pass it to the other Exchange server in the environment as its trying to communicate using the relay connector.
–
Removing the Exchange Server IP from the relay connector of the other exchange server . Restarting the Exchange Transport Service from the non-working Exchange server.
Bringing back the exchange server authentication to work. Mail flow came back to normal
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