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550 5.7.133 error: sender not authenticated for group

Microsoft's 550 5.7.133 means you emailed a group (distribution list) that only accepts mail from inside its organization. Only their admin can change it.

Their mailboxSomething on their side, like a full mailbox. Usually not about you.

What 550 5.7.133 means

Microsoft 365 / Outlook550 5.7.133 Sender not authenticated for group

The group is set to reject mail from outside its organization.

List problem or sender problem?

Their mailbox. It's the group's delivery setting. Only the group's owner or their admin can change it. It's a permanent failure: the server won't accept this message if you send it again unchanged.

How to fix 550 5.7.133

  1. Email a person instead of the group address.
  2. If you need the group, ask its owner or their admin to allow outside senders or add you to the allowed list.
  3. In outreach, suppress group addresses that return this.

In SendingOS, 550 5.7.133 shows on the conversation with its reason and code, so a problem on their side never gets mistaken for one on yours.

Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.

550 5.7.134

Sources: Microsoft: fix 5.7.133 · Microsoft: NDRs in Exchange Online