What Are Null MX Records?
RFC 7505 defines a standardized way for a domain to explicitly indicate that it does not accept inbound email. This is done by publishing a "null MX" record, also known as a positive denial of email delivery.
Using a null MX record improves the sender's experience because delivery fails immediately, rather than timing out after several days. Well-designed email clients may also warn the user that the recipient domain might be incorrect or mistyped.
What a Null MX Record Looks Like
An RFC 7505 null MX record is an MX record with:
Example:
MX 0 .
This must be the only MX record published for the domain (or subdomain).
How It Works
When a sending SMTP server attempts delivery, it checks the domain's MX records. If it finds the null MX record (MX 0 .), it will not fall back to the domain's A/AAAA records. Because the hostname in the MX record is intentionally invalid (empty), the SMTP server has no usable destination for delivery.
Since no other MX records exist, the sending server has no alternative delivery path, and the message will hard-bounce immediately.
Expected Behavior
Properly implemented SMTP servers and mail user agents (MUAs) can recognize this configuration (one MX record with priority 0 and a host of .) and notify the sender that the recipient domain does not accept email. Some clients may also suggest that the domain name could have been entered incorrectly.