Mail Queuing and not being delivered on Ex2016 to EXO

Brass Contributor

Hello. I have an issue with mail building up in our Ex2016 queues. This transports from our 3rd party gateway to Exchange online as part of a hybrid setup. Our mailboxes are all in EXO. Restarting the Transport service clears the queue successfully but it soon builds up again. I'm new to Exchange but on my own here and wonder if anyone can help. Let me know if you need more info. Thanks

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Personally in the scenario you describe where all mailboxes are in Exchange Online, I'd be directing messages from the 3rd party gateway directly to Exchange Online. If you do that you remove your on premises environment as a point of failure, and reduce your network requirements (no more messages coming into your network and then going directly back out to the internet!)
The queue viewer should details why messages are stuck...perhaps there's some resource issues on the Exchange server
Yes, well, in an ideal world. However here I am because $reasons so the dream setup does not yet exist.

Queue viewer did not give any clues that I could use although it is possible I missed something.
This wasn't realy relevant, thank you.

When a Exchange Server 2016 (EX2016) is configured to send emails to Exchange Online (EXO), which is part of Microsoft 365 (formerly Office 365), there can be several reasons why mail queuing occurs and emails are not being delivered. Here are some common explanations for this behavior:

Connectivity Issues: If there is a problem with the network connectivity between the Exchange Server 2016 and Exchange Online, emails may be queued on the Exchange Server and not reach their destination. This could be due to firewall settings, network outages, or DNS configuration problems.

Transport Rule Misconfiguration: If there are transport rules set up in Exchange Server 2016 that interfere with email delivery to Exchange Online, it can lead to mail queuing. For example, a misconfigured rule might redirect or block emails destined for EXO.

Recipient Configuration Errors: Incorrect recipient configurations, such as invalid email addresses or misconfigured forwarding settings, can cause mail queuing. The Exchange Server will keep trying to deliver the emails, but they will remain in the queue until the issue is resolved.

Sender Reputation Issues: If the Exchange Server's IP address or domain has a poor sender reputation, the emails may be blocked or marked as spam by Exchange Online's security measures. This can result in mail queuing or delivery failures.

Large Mail Items or Attachments: Very large emails or attachments can cause mail queuing if the Exchange Server's transport settings or Exchange Online's policies restrict the maximum message size.

Mail Flow Configuration Errors: Incorrect mail flow configuration, such as misconfigured connectors between EX2016 and EXO, can lead to email delivery issues and mail queuing.

Resource Limitations: If the Exchange Server is experiencing resource limitations, such as low disk space or high system load, it may impact its ability to process and deliver emails to Exchange Online.

Hybrid Configuration Issues: If you have set up a hybrid configuration between EX2016 and EXO, any misconfigurations or issues with the hybrid setup can lead to mail queuing and delivery problems.

Temporary Service Outages: Occasionally, both Exchange Server and Exchange Online may experience temporary outages or maintenance periods, leading to mail queuing until the services are restored.

To troubleshoot and resolve mail queuing issues, you can perform the following steps:

Check Connectivity: Verify that there are no network or connectivity issues between EX2016 and EXO. Ensure that both servers can communicate with each other.

Review Transport Rules: Examine the transport rules in Exchange Server 2016 to confirm that they are not interfering with email delivery to EXO.

Verify Recipient and Sender Configurations: Ensure that recipient email addresses are valid and correctly configured. Check for any forwarding or redirection settings that might be causing delivery issues.

Check Sender Reputation: Review the sender reputation of the Exchange Server's IP address and domain to make sure it is not blacklisted or flagged as suspicious.

Inspect Large Mail Items: If large emails or attachments are causing mail queuing, consider adjusting the message size limits in both EX2016 and EXO.

Review Mail Flow Configuration: Double-check the mail flow configuration, connectors, and routing settings between EX2016 and EXO to identify any misconfigurations.

Check Resource Utilization: Monitor the resource utilization on the Exchange Server to ensure it has enough capacity to handle email processing.

Investigate Hybrid Configuration: If you have a hybrid setup, inspect the hybrid configuration for any errors and rectify them.

Monitor Service Status: Keep an eye on the service status of both EX2016 and EXO to check for any temporary outages or maintenance periods.

If you are unable to resolve the issue after troubleshooting, you may need to engage Microsoft support or consult with an experienced Exchange administrator to assist you in identifying and resolving the specific problem causing the mail queuing and delivery delay.

ANSWER: Firewall was blocking connection from Onprem to EXO which was resolving to an IP block we've not seen before. Adding this to the FW rule resolved. I found 451 4.4.397 errors in the SMTP logs to get me there. The IP block was in MS's documentation so was either missed when we set up or added afterwards.
best response confirmed by FuriousHaggis (Brass Contributor)

@FuriousHaggisIt's always the network :lol:

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best response confirmed by FuriousHaggis (Brass Contributor)