RFC 7505 / START HERE
Null MX, before the details
Null MX means no mail exchanger. MX is the DNS mail exchanger record; DNS means Domain Name System.
A domain can publish a special MX record to announce that it accepts no email. Sending mail software can stop promptly instead of trying the domain’s web address as a mail server.
The idea in one picture
Ask for the mail route
Read the no-service record
Stop the mail attempt
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
Without any MX records, SMTP can fall back to the domain’s address records. A website that receives no mail needs an explicit no-service signal to avoid futile attempts and delayed failures.
A familiar example
A website-only domain publishes MX 0 . while keeping its web address. A mistyped email destination then fails promptly; the signal does not authenticate a sender or replace SMTP’s empty return path.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Distinguish no MX from no mailA web address does not announce a mail service. Omitting MX records alone can still make senders try the domain’s address.→
- Read the null MX recordOne ordinary MX record uses preference 0 and the root label as its exchange name.→
- Reject an undeliverable recipient promptlyA submission or relay server can detect null MX while processing a recipient and report permanent failure without a target connection.→
- Keep sender policy and empty return paths separateNull MX concerns receiving mail, so using such a domain as a sender can make replies and delivery-error notifications fail.→
- Keep DNS integrity and service scope distinctNull MX is ordinary DNS data. It can receive DNSSEC protection, but it is not a firewall or a guarantee about unrelated services.→
This introduction explains RFC 7505 (2015). Read the original RFC ↗