550 5.5.0: Mailbox unavailable at Outlook.com or Hotmail

At a glance

• Type: permanent failure (class 5, RFC 3463 §2). The sending server does not retry; sending the same message again fails the same way until the cause is fixed.
• Basic code 550: “Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)” (RFC 5321 §4.2.3).
• Enhanced code 5.5.0: “Other or undefined protocol status” in the IANA registry (RFC 3463), in the group mail delivery protocol status (RFC 3463 §3.6).

Microsoft's consumer mail service could not find the @outlook.com or @hotmail.com address you wrote to. Treat it as a mailbox that does not exist.

What it means

The basic code 550 means "mailbox unavailable", a permanent refusal that covers mailboxes that are not found (RFC 5321 §4.2.3). In the IANA registry, X.5.0 is only "other or undefined protocol status", a catch-all from RFC 3463 §3.6 that says nothing about the mailbox. So the enhanced code alone does not explain this bounce; the domain and the text do.

Microsoft documents 550 5.5.0 for an @hotmail.com or @outlook.com address that its SMTP address lookup cannot find, and describes it as similar to the 550 5.1.10 that Exchange Online returns for business domains (Exchange Online NDR reference). A typical line reads 550 5.5.0 Requested action not taken: mailbox unavailable.

If you see 550 5.5.0 from another provider, read the text after the code. Other servers may use the same catch-all for a protocol problem that has nothing to do with the recipient.

Common causes

• A typo in the part before the @, such as a missing dot or digit.
• The wrong Microsoft domain: outlook.com written where the person uses hotmail.com, or the reverse.
• An account that its owner has closed.
• An address that was never registered, for example one typed into a sign-up form without being confirmed.

How to fix it

1. Check the spelling of the address, including the domain.
2. Ask the person to confirm their address through another channel, or to reply to a message you send from a working channel.
3. In Outlook, delete the old autocomplete entry so the bad address is not offered again.
4. If you collect addresses through a form, use a confirmation email, so addresses that never received anything do not stay on your list.

What to do with the address

Suppress it as a hard bounce and do not retry. Use the address again only if the person confirms it is correct and active. To catch addresses like this before you send, check them with the email checker.

Related codes

• 550 5.1.10: Recipient not found (Microsoft 365)
• 550 5.1.1: The recipient mailbox does not exist
• 5.4.1: Recipient address rejected: access denied (Microsoft 365)

See every bounce code in the dictionary.

Many bounces from addresses that do not exist can be avoided: check an address with our free email checker, or clean a whole list before you send.

How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.