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.”
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