550 5.7.124: sender not in the group's allowed senders

What it means

Distribution groups can be restricted to an allowed senders list. When someone not on it writes to the group, either directly or as a member of a listed group, Exchange Online returns 5.7.124. Depending on how the group is set up, even its owner may need to be on the list to send to it. Inside your organization you can often join the group, after which members can usually send. Otherwise the group owner or an admin has to add you or open the group to everyone; external senders first need a mail contact or mail user representing them.

Common causes

• The group restricts senders to an allowed list that does not include you.
• You are not a member of any group on that list.

How to fix it

1. 01If the group is in your organization, request to join it in Outlook and wait for approval.
2. 02Otherwise ask the group owner, named in the report, to add you to the allowed senders.
3. 03Group owners: add the sender under Delivery management in the Exchange admin center, or allow messages from people inside and outside the organization.
4. 04For an external sender, create a mail contact first, then add it to the list.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• Fix NDR error "550 5.7.124" or "5.7.124" in Exchange Onlinelearn.microsoft.com
• Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com

Further reading

• The standard meaning of 550: mailbox unavailable