5.1.10 SMTP bounce: recipient has null MX

What 5.1.10 means

The recipient domain publishes a null MX record, which says it accepts no mail at all. Delivery cannot succeed.

Message text

Recipient address has null MX

Common causes of 5.1.10

• Recipient domain deliberately publishes a null MX (MX 0 .)

How to fix 5.1.10

1. Run MX lookup on the recipient domain to confirm the null MX.
2. Do not retry; remove 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 5.1.10

The second part, 5.1.10, 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

• Null MX (RFC 7505): publish it for domains with no mail
• How to set up MX records (priorities, null MX, migration)
• MX host does not resolve: causes and how to fix it

Related codes

• 550 5.1.10 Microsoft: recipient not found
• 5.1.1 SMTP: bad destination mailbox
• 5.1.2 SMTP: bad destination system
• 5.1.3 SMTP: bad address syntax
• 550 5.1.1 Microsoft: bad mailbox address
• 550 SMTP: mailbox unavailable

FAQ

All bounce codes