MX record
Where mail for this domain should be handed. A hostname and a preference, not an IP in the MX itself.
MX lists mail exchangers for a domain, each with a preference (lower is tried first). The MX target should be a hostname with A or AAAA, not a CNAME, if you want the internet to behave.
A missing MX does not always mean no mail; some senders fall back to the A record. A stale MX at a decommissioned provider is how you lose mail or let someone else receive it.
See also: A record, CNAME, TXT record, DMARC, DKIM, and SPF.