Microsoft 365 5.7.124
Sender not in allowed-senders list
Microsoft 365 5.7.124 means: sender not in allowed-senders list. 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
Sender not in allowed-senders list
Possible cause
The sender doesn't have permission to send to the distribution group because the sender isn't in the group's allowed-senders list. Depending how the group is set up, even the group's owner might have to be added to the allowed sender list in order to send messages to the group.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.7.124: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.7.0 other or undefined security status. |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
550 5.7.124 means you tried to send to a distribution group that only accepts mail from listed senders. Depending on the group settings, even the group owner may need to be on the list. Ask the group owner or an administrator to add your address. Group owners can also allow external senders in the group settings. The address is valid; resending without permission will keep failing. If you are inside the organisation, ask the owner to add you to the group's allowed senders list rather than sending to members individually.
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 5.7.124?
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 5.7.124 mean?
- Microsoft 365 5.7.124 means: sender not in allowed-senders list. 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 5.7.124?
- 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 5.7.124?
- 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)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05