Null MX: telling the world your domain does not receive mail
11 July 2026 · 3 min read
A domain with no MX still receives: senders fall back to the A record. If nothing listens on port 25 there, mail sits in their queue for days before failing.
example.com. IN MX 0 .
That is a null MX, from RFC 7505. It says definitively that this domain accepts no mail, so senders fail immediately with a clear answer instead of retrying for a week.
When to use it
- Domains you only send from.
- Parked domains and brand defensives.
- Anything where a bounce today beats a mystery on Friday.
More on deliverability
SPF, DKIM and DMARC in plain English
Three records, three different jobs, and one relationship between them that explains almost every "why is my mail in spam" question.
Why forwarded mail lands in spam
Forwarding breaks SPF every single time, by design. What keeps forwarded mail out of the spam folder is DKIM surviving the trip, and that is easy to break.
MTA-STS and TLS-RPT, and when they are worth it
SMTP encryption is opportunistic by default, which means it is trivially downgraded. These two records are the fix, and one of them is nearly free.