MailFixIt

Generic SMTP sender-domain error

SMTP 550 5.7.27: Sender address has null MX

550 5.7.27 means the sender address domain publishes Null MX, which declares that the domain accepts no email. The receiver may reject it because a delivery status notification cannot be returned.

Problem description

A receiver may return:

550 5.7.27 Sender address has null MX

Run this automated test on my domain

Enter your domain. MailFixIt checks the relevant DNS records automatically and shows the evidence.
What MailFixIt can test: whether the current domain publishes valid Null MX and whether ordinary MX records conflict with it. Only the domain owner can confirm whether the no-mail declaration is intentional.
Is this temporary or permanent?

The receiver permanently rejected this message. Send again only after using a suitable sender domain or correcting an accidental Null MX.

Why this happens

How to fix it

  1. Identify the exact sender address named by the response.
  2. Decide whether its domain should receive delivery status notifications.
  3. If it should receive mail, remove Null MX and publish the intended provider's exact MX records.
  4. If Null MX is intentional, configure a suitable envelope sender domain that can receive notifications.
  5. Recheck DNS and send a new test.

Do not replace Null MX with guessed mail servers.

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

Null MX conflicts with ordinary MX recordsDiagnose Null MX email rejection. Learn when MX 0 . is valid, find conflicting ordinary MX records, and repair incoming mail routing safely. SMTP 5.1.8: Bad sender's system addressDiagnose SMTP 5.1.8 when the sender's domain or system address cannot be resolved or accepted. Email delivery delayed or rejected after a DNS migrationDiagnose email that stopped arriving after a DNS change. Compare public MX records with the intended provider and avoid false alarms for mail gateways.

Primary sources: IANA SMTP Enhanced Status Codes registry, RFC 7505 Null MX.