MX record

An MX (Mail Exchange) record is a DNS entry that tells other mail servers which server is responsible for receiving email on behalf of a domain. Each MX record has a priority value — lower numbers are tried first — so a domain can list a primary mail server and one or more backups; Microsoft’s own domains FAQ states this the same way: “mail is delivered to the mail exchange server with the lowest preference number (highest priority).” Google Workspace’s MX setup guide shows a typical case: a single MX record with priority 1 pointing to smtp.google.com, with a note that some registrars require a trailing period on that hostname. Without a correct MX record, incoming mail to that domain has nowhere valid to go and will bounce or queue indefinitely. Like SRV records, MX records point to a hostname, not an IP address directly, so that hostname’s own A/AAAA record also has to resolve correctly for mail delivery to actually complete — RFC 2181 (a Proposed Standard per its IETF Datatracker entry) specifically bars that hostname from being an alias, so when an MX record points to a CNAME covers why that particular setup is invalid rather than just discouraged. A domain that intentionally sends no mail at all should instead publish a null MX record (RFC 7505), which tells sending servers up front not to bother trying, rather than leaving the absence of MX records ambiguous.

MX record is part of AdminKit’s DNS cluster; the full AdminKit glossary lists other related terms.