550 5.5.0 Microsoft bounce: mailbox unavailable
What 550 5.5.0 means
For a Hotmail or Outlook.com recipient, the address was not found during SMTP lookup, so the mailbox is treated as unavailable.
Message text
550 5.5.0 Requested action not taken: mailbox unavailable
Common causes of 550 5.5.0
- Address does not exist at the Outlook.com or Hotmail.com domain
How to fix 550 5.5.0
- Check the recipient address for typos.
- Treat it like 5.1.10 and verify the address exists before sending again.
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How to read 550 5.5.0
The first part, 550, is the SMTP reply code. Requested action not taken, mailbox unavailable: the mailbox does not exist, is not allowed, or the message was refused for policy reasons. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
The second part, 5.5.0, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 5 is mail delivery protocol: a problem in the SMTP conversation itself. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.
Official source: learn.microsoft.com. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 550 5.1.1 Microsoft: bad mailbox address
- 550 5.1.10 Microsoft: recipient not found
- 5.4.1 Microsoft: relay access denied
- 5.4.1 Microsoft: recipient rejected
- 5.4.14 Microsoft: routing loop
- 550 5.7.1 Microsoft: delivery not authorized
FAQ
What does 550 5.5.0 mean?
For a Hotmail or Outlook.com recipient, the address was not found during SMTP lookup, so the mailbox is treated as unavailable. This is a permanent failure.
How do I fix 550 5.5.0?
Check the recipient address for typos. Treat it like 5.1.10 and verify the address exists before sending again.
Is 550 5.5.0 permanent, and should I retry?
550 5.5.0 is a permanent failure, so retrying the same message will fail again. Fix the cause first, then send a new message.