MailFixIt

Outlook SMTP delivery response

Understand Outlook 550 5.5.0: Mailbox unavailable or invalid

The Outlook.com or Hotmail destination address was not found by Microsoft SMTP lookup.

Problem description

A representative response is:

550 5.5.0 Mailbox unavailable or invalid.
Is this temporary or permanent?

This is permanent for the submitted recipient. Correct the address before retrying.

What this response means

Microsoft documents 550 5.5.0 Requested action not taken: mailbox unavailable for an @outlook.com or @hotmail.com address that its SMTP lookup did not find. Other providers can use 5.5.0 differently, so confirm the response came from Microsoft.

Fix it

  1. Confirm the complete recipient address through another channel.
  2. Create a new message and type the corrected address manually.
  3. Remove any stale Outlook autocomplete entry.
  4. If this is a mailing list, suppress the address rather than repeatedly retrying a permanent bounce.
  5. If the owner says the address exists, keep the complete NDR and ask Microsoft support or the account owner to verify its status.

SPF, DKIM, DMARC, and MX changes on the sender’s domain do not create a missing Outlook.com mailbox.

What MailFixIt can check

MailFixIt cannot query Microsoft’s private recipient directory. Its domain checker is unrelated unless the full response also identifies a sender-domain problem.

See the recipient-not-found guides for the same sender and administrator checks.

Frequently asked questions

What does Outlook 550 5.5.0: Mailbox unavailable or invalid mean?

For the Microsoft wording covered here, the Outlook.com or Hotmail recipient address was not found and should be treated like a nonexistent mailbox.

Can MailFixIt prove this entire bounce from public DNS?

No. The current checker can evaluate selected public DNS configuration, but it cannot recreate recipient state, credentials, reputation, message content, or a historical provider SMTP decision.

Related email diagnostics

Outlook 550 5.7.515: Access denied, sending domain does not meet the required authentication levelDiagnose Outlook 550 5.7.515 email rejection safely. Check SPF, DKIM and DMARC DNS, interpret the receiver report, and identify the next configuration step. Outlook.com requirements for high-volume sendersCheck Outlook.com high-volume sender requirements for SPF, DKIM and DMARC alignment, then review sending-IP PTR as separate infrastructure evidence.

Primary sources: Microsoft 365 non-delivery reports.