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Bounce codes › Microsoft 365

550 5.1.10: Recipient not found

Microsoft 365 accepted the connection, looked the address up, and found no mailbox, group or contact with it.

Fix your list Address does not exist

This address was not found at the recipient’s Microsoft 365 organisation.

  • Sent by: Microsoft 365 / Outlook
  • Permanent (hard bounce). Sending the same message again fails again.

How Microsoft 365 words it (source):

Recipient not found

Why it happens

  • A typo in the address.
  • The mailbox was deleted or the address was changed.
  • Outlook filled in an old address from its auto-complete list.
  • A forwarding rule on the recipient side points to an address that no longer exists.

What to do

  1. Type the address again by hand. In Outlook, delete the suggestion that pops up (the X next to it) so the old entry is not used again.
  2. Confirm the address with the person another way.
  3. If it still bounces, remove the address from your list.
  4. If the bounce names a different address from the one you wrote to, the recipient has a broken forwarding rule: tell them.

Don't confuse it with

550 5.4.1 Recipient address rejected: Access denied is the same problem caught earlier. 550 5.1.1 is the same idea from Gmail and most other mail servers.

Questions

Is 550 5.1.10 a hard bounce?

Yes. Treat it as a bad address unless the bounce shows the failure was at a forwarding address.

What does RESOLVER.ADR.RecipientNotFound mean?

It is the name of the step inside Exchange that failed: the address resolver did not find the recipient.

Got a different bounce?

Paste the whole message into the bounce code lookup. It reads the code and the words and tells you whose move it is.

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