Microsoft (Outlook.com and Microsoft 365) · permanent error
550 5.6.240: HVE: MIME header not allowed
The message submitted through HVE contains a MIME header that the service does not permit.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.6.240
- 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 5.6.240 One of MIME Headers is not allowed.
What it means
HVE checks the headers of submitted messages and rejects any message that contains a MIME header it does not permit, with 550 5.6.240. Microsoft's table does not list which headers are refused, so the practical approach is to capture the message the application submits, compare its headers with a message that goes through, and remove the unusual ones.
Common causes
- The application adds a custom or unusual MIME header that HVE does not permit.
- A mail library or template inserts headers the developers did not intend.
How to fix it
- 01Capture the raw message the application submits.
- 02Remove nonstandard or custom headers and test again.
- 03Compare with a message that HVE accepts to narrow down the header at fault.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
Related codes
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce