An MX (Mail Exchange) record maps a domain to one or more hostnames responsible for receiving its mail, each with a priority value; lower priority values are tried first, with higher-numbered records used as fallback if the primary is unreachable.
MX records govern inbound mail routing only. They have no bearing on how a domain's own outbound mail is sent or authenticated; that's SPF, DKIM, and the sending infrastructure's own configuration.
A domain used purely for transactional sending and never for receiving mail should still consider a null MX record (per RFC 7505) to signal explicitly that it accepts no inbound mail, closing off a category of spoofing and misdirected-mail issues.