ProbeKit
Sign inSign up

5.1.10 bounce: recipient domain has null MX

5.1.10Recipient address has null MXPermanent: retrying will not help

Typical wording
556 5.1.10 <user@example.com>: Recipient address rejected: Domain example.com does not accept mail (nullMX)

What it means

X.1.10 was added by RFC 7505 as "Recipient address has null MX". A null MX is a single MX record with priority 0 and a target of just a dot (`example.com. IN MX 0 .`). It is how a domain owner announces that the domain never receives email. RFC 7505 pairs this code with basic reply 556 ("Domain does not accept mail").

The code is usually generated by your own sending server, or by a relay in front of it, as soon as it looks up the recipient domain and finds the null MX. No connection to the recipient is attempted. Microsoft 365 also uses 5.1.10 for a different purpose ("Recipient not found by SMTP address lookup"), which has its own page. Compare 5.7.27, which is the null MX problem on the sender’s domain.

A null MX is a deliberate, definite statement, so the code is always permanent. You will not see a 4.1.10.

Likely causes

  • The address is on a domain used only for a website, parked, or reserved, and the owner published a null MX on purpose.
  • A typo turned the intended domain into a parked domain that happens to have a null MX.
  • The domain owner moved mail elsewhere and published a null MX on the old domain without telling contacts.
  • A DNS template or registrar default added a null MX to a domain that should receive mail.

How to fix it

  1. Check the recipient domain’s MX records. If you see `MX 0 .`, the domain does not take mail and you need a different address.
  2. Look for a typo in the domain and correct it.
  3. If you own the domain and expect mail, replace the null MX with real MX records, for example `example.com. 3600 IN MX 10 mx1.example.net.`, and remove the dot record entirely.
  4. After the change, wait for the old record’s TTL to expire and ask senders to retry.

Checks that apply

  • MX Lookup Shows the null MX record directly, confirming the domain declared that it accepts no mail.
  • DNS Propagation After an owner replaces a null MX, shows whether resolvers worldwide have picked up the new records.
  • Typosquat Scan Helps when a misspelled domain landed on a parked name with a null MX.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

Is a null MX the same as having no MX record?
No. With no MX, senders fall back to the domain’s A or AAAA record. A null MX explicitly forbids delivery, so senders stop immediately.
Why does Microsoft show 5.1.10 for a missing user?
Exchange Online reuses the number for "Recipient not found by SMTP address lookup". The RFC meaning is the null MX described here.

All 191 bounce codes