Definition
MX record (Mail Exchange record): A DNS record naming a server that accepts mail for a domain, with a preference number; senders try the lowest number first. When a domain has no MX record, RFC 5321 has senders fall back to its A or AAAA record.
How MX records work
A sending server looks up the MX records for the recipient's domain, sorts them by preference and connects to the lowest one on port 25. Records with equal preference share the load.
Each MX record must point to a hostname that has its own A or AAAA record. Pointing an MX at a bare IP address or at a CNAME breaks delivery at some senders.
What a null MX record means
RFC 7505 defines a null MX: a single record with preference 0 and a host of just a dot. It says the domain accepts no mail at all.
Senders seeing a null MX fail the message at once instead of queuing it. Domains that only host a website or only send mail publish one to stop bounce traffic and spoofed replies from piling up.
How to check MX records
Run dig MX on the domain, or use an online MX lookup. The output shows each host and its preference.
The hostnames also tell you who runs the mailbox. Google Workspace domains point at Google's mail hosts and Microsoft 365 domains at a host under mail.protection.outlook.com, which tells you what kind of server a verification will be talking to.
$ dig +short MX example.com
10 mx1.example.com.
20 mx2.example.com.
$ dig +short MX parked.example.org
0 .How RocketVerifier handles it
Our free MX lookup lists the mail servers for any domain in priority order. During verification we look up MX once per domain per batch, try up to 3 MX hosts and move on only when a host cannot be reached, and an accept from a backup MX is never trusted as proof. A domain with a null MX or no mail route comes back as undeliverable with sub_status does_not_accept_mail, and a domain that does not exist as invalid_domain.