MX records and priority

An MX record tells senders which mail servers accept email for a domain.

Each MX value contains a preference number and a hostname:

example.com.  3600  IN  MX  10 mail1.example.net.
example.com.  3600  IN  MX  20 mail2.example.net.

Lower numbers are tried first. Here mail2 is a fallback when mail1 cannot be reached; priority 20 does not mean it receives 20 percent of the mail. Equal preferences let senders choose among equally preferred targets.

The target must be a hostname that resolves directly to usable A or AAAA records. Do not put an IP address in the MX value, and do not hide the target behind an HTTP proxy. Mail delivery uses SMTP rather than the web proxy path.

A fallback server is useful only if it is configured to accept and safely relay mail for the domain. An abandoned or weaker fallback can become a security and delivery problem.

Trace the dependency instead of stopping at the MX answer:

dig MX example.com +short
dig A mail1.example.net +short
dig AAAA mail1.example.net +short

An MX query can succeed while delivery fails because the target has no address, port 25 is unreachable, or the server rejects the recipient. DNS evidence narrows the problem but does not test the complete SMTP exchange.

If a domain intentionally accepts no email, the standardized null MX form is preference 0 with target .. Do not invent a fake mail hostname for that purpose.

Your action is to inspect one domain’s MX set. Sort the targets by preference, resolve each target’s addresses, and explain which failure would cause a sender to try the next server.