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How to Fix Exchange Online Error 550 5.0.350 in Outlook
Question details
The user is experiencing an intermittent Exchange Online non-delivery report (NDR) error 550 5.0.350, stating that a mailbox does not exist, particularly when emails pass through external gateways.
- Product
- Exchange Online
- Device & OS
- not provided
- Scenario
- Sending emails that route through external third-party gateways such as Exclaimer.
- Observed behavior
- Messages intermittently fail to deliver, returning a 550 5.0.350 bounce-back error indicating the recipient mailbox does not exist.
Ensure you have Exchange administrator permissions to run a message trace and have the complete non-delivery report (NDR) text available for review.
Review the Message Trace and NDR
Analyze the Exchange routing logs to identify exactly where the message delivery fails.
Because this error is intermittent and involves external gateways, pinpointing the exact failure point is crucial. The message trace will reveal if the message is being dropped before, during, or after handoff to the external gateway.
Log into the Microsoft 365 Exchange admin center using your administrator credentials.
Navigate to 'Mail flow' and select 'Message trace'. Enter the sender and recipient details to track the affected email.
Review the routing hops in the trace and compare them with the complete non-delivery report (NDR) to see if the gateway (e.g., Exclaimer) is rejecting the message.
Send Messages in HTML Format
Changing the sending format to HTML can prevent the message from being treated as a nested attachment or blocked by transport policies.
Consult Microsoft Support with Diagnostic Details
Escalate the issue to Microsoft Support if the error persists after formatting and routing checks.
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Frequently Asked Questions
What does Exchange error 550 5.0.350 mean?
Error 550 5.0.350 is a non-delivery report (NDR) indicating that the recipient mailbox does not exist or the message was rejected by the destination server. It frequently occurs when emails pass through external gateways.
Why do external gateways like Exclaimer trigger this error?
External signature gateways modify the email in transit. If Exchange transport rules are misconfigured or the format changes unexpectedly, the receiving server might reject the modified message, triggering the 5.0.350 error.
Does changing the email format to HTML fix error 5.0.350?
Yes, in many instances. Sending the message strictly in HTML format prevents external gateways from misinterpreting the email body as a nested attachment or violating strict transport policies during the handoff.
Where can I find the complete non-delivery report (NDR)?
The complete NDR is typically found in the bounce-back email received by the sender. It contains technical diagnostic details, error codes, and the original message headers necessary for troubleshooting.
