550 5.0.350: generic error from the recipient's system
Exchange Online wraps an unspecific rejection from the recipient's mail system in 5.0.350; the real reason is in the original reply quoted in the report.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.0.350
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
What it means
5.0.350 is a catch-all. Exchange Online uses it when the recipient's organization rejected the message with an error that does not map to a more specific code. The useful part of the bounce is the diagnostic section for administrators, which quotes the response the external server or service gave. Microsoft adds that its support usually cannot help, because the problem sits in the recipient's system. Two variants have their own fixes: x-dg-ref header is too long, and policy violation detected (AS345).
Common causes
- The recipient's server rejected the message for a reason of its own, quoted in the report.
- An Outlook message in Rich Text format carried a large nested attachment (the x-dg-ref variant).
- An attachment contained 20 or more embedded files (the AS345 variant).
How to fix it
- 01Open the report and read the original error in the diagnostic information for administrators.
- 02If it mentions x-dg-ref or AS345, follow the fix for that variant.
- 03Otherwise forward the report to your admin, who may need to contact the recipient's organization with the quoted error.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Fix NDR error "550 5.0.350" in Exchange Onlinelearn.microsoft.com
- Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
Related codes
- 550 5.0.350Rich Text attachment makes a header too long.A message in Outlook's Rich Text format carried an attachment whose data ended up in a header line too long for the receiving server.
- 550 5.0.350 [AS345]Attachment with too many embedded files.The message was rejected because an attachment contains 20 or more embedded files, which is treated as a policy violation.
- 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.
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