550 5.7.124: sender not in the group's allowed senders list
You sent to a distribution group in Microsoft 365 that only accepts messages from specific senders, and you aren't on its allowed senders list.
Sender not in allowed-senders list
Microsoft's description of 5.7.124 in its Exchange Online NDR reference. The NDR you receive also explains how to find the group's owner.
What 550 5.7.124 means
Microsoft says you'll see 550 5.7.124 when the sender isn't specified in the group's allowed senders list, either directly or as a member of a group that is. Depending on how the group is configured, even the group's owner might need to be on the list to send to it.
This is a delivery restriction on one group, not a spam verdict or a block on your domain. Your mail to other recipients is unaffected. The group's owner or an email admin chose to restrict who can post, which is common for company-wide lists, board lists and announcement groups.
Microsoft adds that groups with more than 5,000 members automatically have extra restrictions: senders must be members of the group, and messages need a moderator's approval. Oversized messages to such groups get a different NDR.
If you're inside the same organization, the quickest fix may be to join the group in Outlook or Outlook on the web; you might have to wait for the owner to approve the request. If you're outside the organization, or the group doesn't accept join requests, ask the owner to add you. Microsoft notes that an outside sender can only be added after an admin creates a mail contact or mail user to represent them in the organization.
Likely causes
- You aren't on the group's allowed senders list, directly or through another group.
- You're the group's owner, but the group's settings still require owners to be listed.
- The group has more than 5,000 members, so only members can send and messages need moderator approval.
- You're an outside sender, and nobody has created a mail contact for you in the recipient's organization.
How to fix it
If you sent the message
- If the group is in your organization, try to join it in Outlook or Outlook on the web, then wait for the owner's approval.
- Otherwise, ask the group owner to add you. The NDR explains how to find the owner.
- If it's urgent, send to the individual people instead of the group.
If you're the recipient or their admin
- In the Exchange admin center, go to Recipients > Groups, open the distribution list, select Settings > Edit delivery management, and add the sender under Specified senders.
- For an outside sender, first create a mail contact or mail user that represents them, then add it to the allowed senders list.
- If the group doesn't need restrictions, choose "Allow messages from people inside and outside my organization" under Sender options.
Frequently asked questions
Why does a group owner get 550 5.7.124 when emailing their own group?
Microsoft says that, depending on how the group is configured, even the owner might need to be on the group's allowed senders list. Add the owner, or a group the owner belongs to, under Specified senders in the group's delivery management settings.
Can an outside sender be added to a distribution group's allowed senders list?
Yes, but not by email address alone. An admin first creates a mail contact or mail user that represents the outside sender in the organization, then adds that object to the group's allowed senders list.
Sources
- Fix NDR error 5.7.124 in Exchange Online (Microsoft Learn)
- Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn)
Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.
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