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How to Change the Exchange Online SMTP Timeout for Large Messages
Question details
The user needs to increase or configure the SMTP timeout setting in Exchange Online when sending large messages with attachments to a third-party server.
- Product
- Exchange Online
- Device & OS
- not provided
- Scenario
- Sending large messages containing images or attachments to a third-party SMTP server.
- Observed behavior
- Exchange Online times out and disconnects before receiving delivery confirmation, causing it to retry delivery even if the partner server already accepted the message.
Ensure you have Microsoft 365 Administrator privileges, as investigating and resolving backend SMTP timeout issues requires access to the Exchange admin center and the ability to open support tickets.
Run an Extended Message Trace and Contact Microsoft Support
Since the SMTP timeout is not a user-configurable setting, administrators must investigate the issue using a message trace and open a service request with Microsoft.
Microsoft restricts direct access to SMTP timeout configurations in Exchange Online to maintain server performance and security. Therefore, the timeout threshold for third-party connections is not exposed in the Microsoft 365 admin center or the Exchange admin center.
To resolve ongoing timeout issues with specific partner servers, administrators must capture diagnostic data about the delivery failures and provide it to Microsoft Support for a backend resolution.
Sign in to the Microsoft 365 admin center using your global or exchange administrator credentials.
Navigate to the Exchange admin center, access the 'Mail flow' section, and run an 'Extended message trace'. This will capture detailed logs of the delivery attempts and connection drops involving the third-party SMTP server.
Return to the Microsoft 365 admin center, navigate to the Support menu, and create a new service request. Attach the extended message trace results to help the support team investigate.
Alternatively, you can call Microsoft Support directly using the official global customer service phone numbers for your region to expedite the request.
Need to Create Smaller Attachments? Try WPS Office
While Exchange Online manages your backend email delivery, handling large attachments can still be a challenge. WPS Office is a seamless, lightweight alternative to Microsoft Office that includes built-in PDF compression and efficient file formats, reducing the chance of email timeouts.
- 1. Download the Installer: Visit the official WPS Office website and click the Free Download button.
- 2. Install WPS Office: Run the downloaded setup file and follow the quick on-screen instructions to install the suite.
- 3. Compress and Send: Open your large documents, use the built-in file compression or PDF export tools, and attach the smaller files to your emails without worrying about SMTP timeouts.
Frequently Asked Questions
Can I manually change the SMTP timeout limit in Microsoft 365?
No. The SMTP timeout threshold for third-party connections is not exposed as a configurable setting in the Microsoft 365 admin center or the Exchange admin center.
Why does Exchange Online send duplicate messages to a partner server?
If the connection drops or times out before Exchange receives a successful delivery confirmation, it assumes the email was not delivered. It will then retry sending the message, even if the partner server had already accepted the initial payload.
What is an extended message trace used for?
An extended message trace provides in-depth diagnostic logs about routing and message delivery attempts. Microsoft Support uses these detailed logs to investigate and resolve underlying server communication or timeout issues.
