When Office 365 tried to send your message, the receiving email server outside Office 365 reported an error. 550 5.0.350 Remote server returned an error

Question

Hi Support

When Office 365 tried to send your message, the receiving email server outside Office 365 reported an error. 550 5.0.350 Remote server returned an error. Please advise

Answer 1

Hi Chung Lin (Airlink)

Thank you for reaching out to the Microsoft Q&A Forum and for sharing your issue.

Based on my research, the error 550 5.0.350 is a generic non-delivery report (NDR) generated by Exchange Online when the receiving email server returns a non-specific error. In most cases, this means the message was successfully sent from Microsoft 365 but was rejected by the recipient’s email system.

Please open the full NDR (bounce message) and review the section “Diagnostic information for administrators”, as it should contain a more specific error returned by the receiving Server.

If the NDR includes specific messages such as:

-“x-dg-ref header is too long”: this may occur when using Rich Text format with complex attachments. Switching to HTML format and resending the email may help.

-“Requested action not taken: policy violation detected (AS345)”: this can happen when an attachment contains too many embedded files. Reducing the number of embedded items may resolve the issue.

If none of the above specific cases are mentioned, could you please share the full NDR with me via private message to avoid exposing any personal or organizational information? This will help me better understand your issue.

For more information, you can refer to the documentation: https://learn.microsoft.com/troubleshoot/exchange/email-delivery/ndr/fix-error-code-550-5-0-350-in-exchange-online

I hope this helps.

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