MX record
The MX record designates the servers that receive mail for a domain, along with their priorities. Without a valid MX, inbound messages bounce — regardless of the website status.
In practice
MX points to a hostname, never a direct IP, and that hostname must resolve cleanly. Priorities organize failover: the lowest number is tried first. After any change, an end-to-end test — a real send, a real receipt — remains the only verdict that matters.