An MX record is a publicly available address for an email recipients mail server. When someone, for example:firstname.lastname@example.org sends an email to email@example.com, the sender’s mail transfer agent looks up the available MX record at public DNS registry of xyz.com to determine where to deliver emails.
A domain/ sub-domain cannot have multiple MX records. However, they can have multiple hosts associated with an MX record organized by priority.
MX record is mandatory to avoid failing anti-virus checkers against the registered domain/sub domain for this service. The MX record is also needed for “reply” tracking. The “reply to” address allows a recipient to respond back to an email. When a marketing / transactional email is replied to, the mail transfer agent looks up DNS entries to determine the MX record of the recipient’s mail server.