An MX record is a DNS record that tells the world — specifically, every mail server on the planet — where to deliver email for your domain. Without it, mail bounces.
The shape
example.com. IN MX 10 mail1.example.com.
"10" is the priority — lowest wins. If mail1 is unreachable, mail tries the next-lowest-priority MX. "mail1.example.com" is the hostname of the receiving mail server; that hostname needs its own A record.
What can go wrong
- No MX record. Mail bounces with "no MX record for domain."
- MX points at a CNAME. Forbidden by RFC. Some servers tolerate it; some reject.
- MX hostname has no A record. Receiving servers can't connect.
- MX server isn't reachable. Firewall blocking port 25 inbound, mail queue full, etc.
Our MX Lookup reports all of the above and also surfaces the SPF + DMARC posture in the same response.