550 5.0.350: rich 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.
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- Permanent (5xx)
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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
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
550 5.0.350 x-dg-ref header is too long
What it means
Outlook's Rich Text format carries attachment data, as a binary large object, inside the X-MS-TNEF-Correlator header. With a large attachment, often an attached email that itself contains another attached email, that header line grows longer than the receiving server accepts, and it rejects the message. Exchange Online reports it as 550 5.0.350 with the remote reply x-dg-ref header is too long. Sending the message in HTML keeps the attachment out of the TNEF header.
Common causes
- The message was composed in Rich Text format in Outlook.
- It carried an attachment, often an email message with another message attached inside it.
How to fix it
- 01Switch the message format to HTML in Outlook and send it again.
- 02Make HTML the default format for new messages to avoid repeats.
- 03Admins can control TNEF use for external recipients in remote domain settings or on mail contacts and mail users.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- "550 x-dg-ref header is too long" when sending mail in Exchange Onlinelearn.microsoft.com
- Fix NDR error "550 5.0.350" in Exchange Onlinelearn.microsoft.com
Related codes
Further reading
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