Microsoft 365 550 5.1.20
Multiple From addresses are not allowed without Sender address
Microsoft 365 550 5.1.20 means: multiple From addresses are not allowed without Sender address. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Multiple From addresses are not allowed without Sender address
Possible cause
An email message has multiple email addresses in the From field, but no email address in the Sender field.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | 550: Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons) (RFC) |
| Enhanced code | 5.1.20: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.1.0 other address status. |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
What the sender should do
The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Would verifying the list have prevented Microsoft 365 550 5.1.20?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Microsoft 365 550 5.1.20 mean?
- Microsoft 365 550 5.1.20 means: multiple From addresses are not allowed without Sender address. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Should I retry after Microsoft 365 550 5.1.20?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Would verifying my list have prevented Microsoft 365 550 5.1.20?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05