550 5.7.193: microsoft 365 group does not accept external mail

What it means

A group in Microsoft 365 Groups refuses mail from outside senders unless it is set to accept them or the sender is a member. An external sender who falls into neither case gets 550 5.7.193 UnifiedGroupAgent. The group owner can add that person as a guest member, or let people outside the organization email the group, in Outlook. An admin can do the same in the Exchange admin center, or with Set-UnifiedGroup -RequireSenderAuthenticationEnabled $false in Exchange Online PowerShell.

Common causes

• The group is not set to receive mail from people outside the organization.
• The sender is not a member of the group, as a guest or otherwise.

How to fix it

1. 01Ask the group owner to allow your address.
2. 02Group owners: in Outlook, edit the group and either add the sender as a guest member or turn on Let people outside the organization email the group.
3. 03Admins: in the Exchange admin center, select Allow external senders to email this group, or run Set-UnifiedGroup -Identity <group> -RequireSenderAuthenticationEnabled $false.

Sources

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

• NDR 550 5.7.193 when external users send email to a Microsoft 365 grouplearn.microsoft.com

Further reading

• The standard meaning of 550: mailbox unavailable