Null MX record

Basically the “null MX” protocol is a simple mechanism by which a domain can indicate that it does not accept email. It is described in RFC 7505.

This protocol defines a null MX that will cause all mail delivery attempts to a domain to fail immediately.

DNS record

A “null MX” RR is an ordinary MX record with an RDATA section consisting of preference number 0 and a “.” as the exchange domain, to denote that there exists no mail exchanger. 86400 IN MX 0 "."

A domain that advertises a null MX MUST NOT advertise any other MX RR.

Sender with “Null MX” records

As Null MX is primarily intended for domains that do not send or receive any mail, vSMTP default behavior is to reject mail that has an invalid return address.

Mail systems should not publish a null MX record for domains that they use in MAIL FROM (RFC5321) or From (RFC5322) directives.

Null MX return codes

The RFC 7505 defines two specific return codes.

TextRecipient address has null MX
Basic status code556
DescriptionThe associated address is marked as invalid using a null MX.
TextSender address has null MX
basic status code550
DescriptionThe associated sender address has a null MX, and the SMTP receiver is configured to reject mail from such sender (e.g., because it could not return a DSN).

vSL predefined functions

This is a DRAFT for v1.2 features

The standard API as a dedicated abstract to check the Null MX record.

// -- main.vsl

mail: [
  rule "check null MX" || check_null_mx();

Current Behavior

Currently, MX records are automatically checked on delivery. If the destination server provide a NULL MX record, the email is immediately placed into the dead queue.