550 5.7.133: group rejects mail from outside the organization

What it means

5.7.133 tells an external sender that the group they wrote to only takes mail from inside its organization. Distribution groups can require senders to be authenticated, which in practice means internal. Only the group owner or an email admin of that organization can change this, either by allowing messages from people inside and outside the organization, or by adding particular external senders, represented by mail contacts or mail users, to the group's allowed senders list.

Common causes

• The group is set to accept messages only from people inside the organization.
• The group is meant for internal use and outside senders are deliberately excluded.

How to fix it

1. 01Contact the group owner or the organization's admin and ask whether you can be allowed.
2. 02Group owners: under Delivery management, allow messages from people inside and outside the organization.
3. 03Or keep the restriction and add specific external senders to the allowed senders list as mail contacts.

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 5.7.133" 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