Mail Exchange (MX) Record

A mail exchange record (MX record) is a type of certified and verified resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient’s domain, and a preference value used to prioritize mail delivery if multiple mail servers are available.

The set of MX records of a domain name specifies how email should be routed with the Simple Mail Transfer Protocol (SMTP). A great site for testing the different aspects of an mx record including blocklists is https://www.mxtoolbox.com.