Intermittent hybrid mail flow latency in Exchange 2019 and Exchange Online environment

Hi @Slater Kinja

Thank you for posting your question in Microsoft Q&A. 

We understand that you are experiencing intermittent delays in your hybrid environment when sending mail from on-premises mailboxes to cloud mailboxes. This issue may be caused by throttling, backpressure, or temporary resource constraints on the Exchange Online (EXO) side. However, they could also stem from configuration mismatches, certificate issues, or restrictions Microsoft imposes on mail originating from vulnerable on-premises servers.  

To help determine where the delay originates, you could try to follow this approach: Enable and Analyze Protocol Logging on the Outbound Send Connector. 

This is the most effective method to capture SMTP conversations at the connector level, including detailed rejection messages from Exchange Online. It can help determine whether the issue stems from throttling, TLS/certificate problems, or other remote responses. 

• Identify your hybrid outbound Send connector:

    Get-SendConnector | FL Identity,Name. 
  

• Enable verbose logging:

    Set-SendConnector -Identity "YourHybridOutboundConnectorName" -ProtocolLoggingLevel Verbose. 
  

• Restart the Microsoft Exchange Transport service:

    Restart-Service MSExchangeTransport. 
  

Log Location: %ExchangeInstallPath%TransportRoles\Logs\Hub\ProtocolLog\SmtpSend 

After observing a delay, open the most recent log file in the SmtpSend folder. Review the full SMTP conversation to identify where the delay or rejection occurs. 

If the log shows successful initiation from the on-premises server but repeated rejections from the remote server, the issue likely lies with Exchange Online (throttling). If there’s no connection attempt or failure occurs locally, the issue is likely on the on-premises side. 

Additionally, I have found that this behavior might be caused by default Exchange Server settings that limit the volume of messages sent to a single destination, such as Exchange Online. 

To troubleshoot it, you could try to: 

Reconfiguring SmtpConnectorQueueMessageCountThresholdForConcurrentConnections parameter 

Increase MaxPerDomainOutboundConnections 

Reduce the retry interval value 

For more information, you could refer to: Email messages are stuck in Exchange Server queues for several minutes - Exchange | Microsoft Learn 

Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution. 

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