550 5.5.0 Outlook.com: mailbox unavailable
Quick answer
From Outlook.com, Hotmail and Live, this reply means the mailbox does not exist. Microsoft's consumer service uses the RFC 5321 default wording, "Requested action not taken: mailbox unavailable", instead of a "user unknown" message.
Recommended action: Suppress the address
Common causes
- The Outlook.com or Hotmail account was closed, often after long inactivity.
- A typo in the local part.
- A made-up address entered in a form.
What to do
Suppress the address. On Outlook.com the reply is a hard bounce.
Provider note: On other servers the same wording can be a policy rejection, so RocketVerifier only reads it as mailbox_not_found on Microsoft hosts and treats it as ambiguous (unknown, free) everywhere else.
How to read 550 5.5.0
550 is the basic SMTP reply defined in RFC 5321. 5.5.0 is the enhanced status code from RFC 3463, which reads as class, subject and detail:
- Class
- 5Permanent failure
- Subject
- .5Mail delivery protocol
- Detail
- .0Outlook.com: mailbox unavailable
Further reading
FAQ
Frequently asked questions
Why do some checkers return unknown for Hotmail addresses?
Because "mailbox unavailable" is ambiguous without knowing the provider. A verifier without Microsoft-specific rules has to treat it as unknown.
Stop hard bounces before they start
Upload a list or call the API, and send only to addresses that can receive mail. Unknown results are always free.
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