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Help · Microsoft 365

550 5.7.133 — your distribution group rejects external senders

550 5.7.133 RESOLVER.RST.SenderNotAuthenticatedForGroup
The recipient isn't authorized to receive messages from unauthenticated senders. Delivery has failed.

What's actually happening

New distribution groups are created with RequireSenderAuthenticationEnabled set to true, which means only people inside your organisation can email the group. Customers emailing sales@ or info@ get this bounce and you never see their message.

The fix — admin center

  1. Open admin.exchange.microsoft.com → Recipients → Groups and select the group.

  2. Open Settings and find Allow external senders to email this group.

  3. Turn it on and save. The change is usually immediate.

The fix — PowerShell, if you prefer

Set-DistributionGroup -Identity "info@yourdomain.com" ` -RequireSenderAuthenticationEnabled $false

Still not working?

If external mail still bounces, check the group's accepted senders list is empty (a non-empty list overrides the setting), and run a message trace to see which policy rejected it.

If you're doing this to share one address across a team: that's what Bosun is for — one forwarding rule, and everyone answers hello@ from their own login.