451 4.4.0 Primary target IP address responded with: "454 4.7.0 Temporary authentication failure." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
Additionally, you may find the following error message in the Application log file on the Exchange server that is receiving the e-mail message:
Event Type: Error Event Source: MSExchangeTransport Event Category: SmtpReceive Event ID: 1035 Description: Inbound authentication failed with error IllegalMessage for Receive connector Default <Server> . The authentication mechanism is ExchangeAuth. The source IP address of the client who tried to authenticate to Microsoft Exchange is [xxx.xxx.xxx.xxx].
This issue occurs if the Exchange server cannot authenticate with the remote Exchange server. Exchange servers requires authentication to route internal user messages between servers. The issue can be caused by one of the following reasons:
The Exchange server is experiencing Time synchronization issues
There is a replication issue between the domain controllers
The Exchange server is experiencing Service Principal Name (SPN) issues
The required TCP/UDP ports for the Kerberos protocol are blocked by the firewall
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