stackexchange-dnscontrol/documentation/functions/domain/MX.md

name	parameters	parameter_types
MX	name priority target modifiers...	name priority target modifiers... string number string RecordModifier[]

MX adds an MX record to the domain.

Priority should be a number.

Target should be a string representing the MX target. If it is a single label we will assume it is a relative name on the current domain. If it contains any dots, it should be a fully qualified domain name, ending with a ..

D("example.com", REGISTRAR, DnsProvider(R53),
  MX("@", 5, "mail"), // mx example.com -> mail.example.com
  MX("sub", 10, "mail.foo.com.")
);