5.7.27 SMTP bounce: sender has null MX
What 5.7.27 means
The sender's domain publishes a null MX, and the receiver rejects mail from senders with a null MX.
Message text
Sender address has null MX
Common causes of 5.7.27
- Envelope sender domain publishes a null MX record
How to fix 5.7.27
- Run MX lookup on the sender domain.
- If you want replies or bounces, publish a real MX record for the sending domain.
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 5.7.27
The second part, 5.7.27, 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 7 is security or policy: authentication, authorization or policy refused the message. 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 create an SPF record (syntax and examples)
- SPF ~all vs -all vs ?all: softfail, fail and neutral
- How DKIM works and how to publish a DKIM record
Related codes
- 550 5.7.27 Gmail: SPF not passed
- 5.7.0 SMTP: security status
- 5.7.1 SMTP: delivery not authorized
- 5.7.23 SMTP: SPF validation failed
- 5.7.25 SMTP: reverse DNS failed
- 5.7.26 SMTP: authentication failed
FAQ
What does 5.7.27 mean?
The sender's domain publishes a null MX, and the receiver rejects mail from senders with a null MX. This is a permanent failure.
How do I fix 5.7.27?
Run MX lookup on the sender domain. If you want replies or bounces, publish a real MX record for the sending domain.
Is 5.7.27 permanent, and should I retry?
5.7.27 is a permanent failure, so retrying the same message will fail again. Fix the cause first, then send a new message.