5.1.1 SMTP bounce: bad destination mailbox

What 5.1.1 means

The recipient mailbox does not exist. The address is wrong or the mailbox is gone, so retrying will not help.

Message text

Bad destination mailbox address

Common causes of 5.1.1

• Typo in the recipient address
• Mailbox was deleted or renamed

How to fix 5.1.1

1. Verify the address with the recipient.
2. Remove the address from your list; do not retry.

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 5.1.1

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: www.rfc-editor.org. 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

• 550 5.1.1 Microsoft: bad mailbox address
• 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
• 550 SMTP: mailbox unavailable

FAQ

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