5.1.2 SMTP bounce: bad destination system

What 5.1.2 means

The destination system named in the address does not exist or cannot accept mail.

Message text

Bad destination system address

Common causes of 5.1.2

• Recipient domain has no working MX or address record
• Domain misspelled in the recipient address

How to fix 5.1.2

1. Run MX lookup on the recipient domain.
2. Check the domain spelling in the address.

Check your domain

Check the MX records of the recipient domainRuns 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 5.1.2

The second part, 5.1.2, 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

• MX host does not resolve: causes and how to fix it
• How to set up MX records (priorities, null MX, migration)
• MX record points to a CNAME: why it breaks and the fix

Related codes

• 5.1.1 SMTP: bad destination mailbox
• 5.1.3 SMTP: bad address syntax
• 5.1.10 SMTP: recipient has null MX
• 550 5.1.1 Microsoft: bad mailbox address
• 550 5.1.10 Microsoft: recipient not found
• 550 SMTP: mailbox unavailable

FAQ

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