Also called mail exchanger record
MX record, explained
Sending servers look up the recipient domain's MX records and try them in priority order, moving to the next if one is unreachable. If a domain has no MX record, senders fall back to the addresses in its A or AAAA records. RFC 7505 defines a null MX, a single MX record with priority 0 and a target of a single dot, which states that the domain accepts no mail.
On names you hold for sale or parking, publish a null MX, together with an SPF record that authorizes no senders and a DMARC policy of reject, so the domain is harder to use in spoofed email. On expired names you buy, mail meant for the previous owner, such as invoices or password resets, may keep arriving once mailboxes exist; do not set up a catch-all to collect it, and check how the name was used before you buy.
Example. Before listing the domain for sale, the owner deleted the old MX records and published a null MX so the name would stop accepting mail.
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Education, not financial, legal or tax advice. Domain investing is speculative and most domain names never sell. Read the disclaimer.