Generic SMTP sender-domain error
SMTP 5.7.27: sender address has Null MX
5.7.27 means the sender address domain publishes Null MX—an explicit declaration that it does not accept email. That can be intentional, but it conflicts with sending paths expected to receive replies or delivery notifications.
A receiver may return:
550 5.7.27 Sender address has null MX
Check the sender domain's MX state
The 5.x.x result is permanent for this message. A new attempt is appropriate only after using a sender domain with correct receiving intent or correcting an accidental Null MX.
Why this happens
- The sender or return-path domain intentionally uses
MX 0 .. - A no-mail record remained after the domain began sending.
- The sending service selected the wrong envelope domain.
- Null MX is mixed with ordinary MX records, creating contradictory intent.
How to resolve it
- Identify the exact sender address named by the response.
- Confirm whether that domain should receive replies or DSNs.
- If receiving mail is intended, configure the provider's exact MX set and remove Null MX as one controlled change.
- If no-mail intent is correct, configure the sending service to use an appropriate sender/return-path domain.
- Recheck DNS and send a new test.
Do not replace Null MX with guessed mail servers. MX values must come from the intended receiving provider.
Frequently asked questions
What does SMTP 5.7.27 mean?
The sender address domain publishes Null MX, explicitly stating that it accepts no email.
Is Null MX always an error?
No. It is valid for a domain intentionally unable to receive email. It conflicts with a sender address when replies or delivery status notifications are expected.
Can I keep Null MX and send email?
A provider may reject such mail because the sender domain cannot receive notifications. Use a suitable return-path domain that can receive mail when required.
Related email diagnostics
Primary sources: IANA SMTP Enhanced Status Codes registry and RFC 7505 Null MX.