550 5.1.1 Microsoft bounce: bad mailbox address
What 550 5.1.1 means
Exchange Online could not find a mailbox for the recipient address. The address is wrong or has changed.
Message text
Bad destination mailbox address
Common causes of 550 5.1.1
- Sender typed the recipient address incorrectly
- Recipient mailbox was moved or deleted
- Outlook recipient cache on the sender side holds a stale address
How to fix 550 5.1.1
- Check the recipient address for typos.
- If both users are in the same organization, remove the address from the sender's Outlook recipient cache and send a new message.
- Confirm the recipient mailbox exists with the recipient domain's administrator.
Check your domain
Read the headers of a bounced messageRuns in your browser; nothing is uploaded.
Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.
How to read 550 5.1.1
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, 5.1.1, 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 1 is addressing: a problem with the recipient or sender address. 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 set up MX records (priorities, null MX, migration)
- MX host does not resolve: causes and how to fix it
- Null MX (RFC 7505): publish it for domains with no mail
Related codes
- 5.1.1 SMTP: bad destination mailbox
- 5.1.2 SMTP: bad destination system
- 5.1.3 SMTP: bad address syntax
- 5.1.10 SMTP: recipient has null MX
- 550 5.1.10 Microsoft: recipient not found
- 5.4.1 Microsoft: relay access denied
FAQ
What does 550 5.1.1 mean?
Exchange Online could not find a mailbox for the recipient address. The address is wrong or has changed. This is a permanent failure.
How do I fix 550 5.1.1?
Check the recipient address for typos. If both users are in the same organization, remove the address from the sender's Outlook recipient cache and send a new message. Confirm the recipient mailbox exists with the recipient domain's administrator.
Is 550 5.1.1 permanent, and should I retry?
550 5.1.1 is a permanent failure, so retrying the same message will fail again. Fix the cause first, then send a new message.