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550 5.7.134 error: sender not authenticated for mailbox

Microsoft's 550 5.7.134 means the recipient's mailbox only takes mail from authenticated, usually internal, senders. Only their admin can change it.

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

What 550 5.7.134 means

Microsoft 365 / Outlook550 5.7.134 Sender was not authenticated for mailbox

The mailbox rejects mail from external or unauthenticated senders.

List problem or sender problem?

Their mailbox. It's the mailbox's delivery setting. Only 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 5.7.134

  1. Reach the person another way: you can't change their setting.
  2. Their admin can turn off "Require senders to be authenticated" or add you to the allowed senders.
  3. In outreach, suppress the address as unreachable.

In SendingOS, 550 5.7.134 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 5.7.133

Sources: Microsoft: fix 5.7.134 · Microsoft: NDRs in Exchange Online