5.7.27 bounce: sender address has null MX
5.7.27Sender address has null MXPermanent: retrying will not help
556 5.7.27 <noreply@example.com>: Sender address rejected: Domain example.com does not accept mail (nullMX)
What it means
RFC 7505 (A “Null MX” No Service Resource Record) defines X.7.27, “sender address has null MX”. A null MX is a single MX record with preference 0 and a target of “.”, and it announces that the domain accepts no mail. A receiver may reasonably refuse mail whose sender domain cannot accept the bounces and replies it might generate.
This code is about the sender side. Its mirror, 5.1.10 (“recipient address has null MX”), is what you get when you write to a domain that published a null MX. Postfix, for instance, refuses a null-MX sender domain with 5.7.27 when `reject_unknown_sender_domain` is enabled, using its `nullmx_reject_code` (556 by default).
Because the record is a deliberate statement, there is no temporary form in practice. The receiver will keep refusing until the domain owner changes its MX or the sender uses a different envelope address.
Likely causes
- A domain set up for web only that publishes `example.com. MX 0 .` but is then used to send mail.
- An application that sends with a noreply address on a parked or null-MX domain.
- A migration that left a null MX in place after the domain started being used for email again.
- A subdomain used as the envelope sender that has a null MX inherited from a template.
How to fix it
- Look up the MX record of the domain in your envelope sender (the Return-Path address).
- If the domain should send mail, replace the null MX with real MX records that can accept bounces, for example `example.com. MX 10 mx1.example.com.`.
- If the domain should not receive mail, change the application to use an envelope sender on a domain that does have working MX.
- Send a test after DNS has propagated and confirm the receiver accepts MAIL FROM.
Checks that apply
- MX Lookup Shows whether the sender domain publishes a null MX (preference 0, target “.”), the direct cause of 5.7.27.
- DNS Propagation Confirms resolvers worldwide see your corrected MX before you retry.
FAQ
- What is the difference between 5.7.27 and 5.1.10?
- 5.7.27 is about the sender’s domain having a null MX; 5.1.10 is about the recipient’s domain having one.
- Can I keep a null MX and still send?
- Use a different envelope sender domain that accepts bounces. Many receivers will refuse a null-MX sender domain no matter how well the rest is configured.