Resources
What is an MX record?
The MX record says which servers accept mail for a domain, and in which order.
Each MX record carries a priority and a hostname. Lower numbers are tried first; equal numbers share the load.
The hostname in an MX must resolve to an address record. Pointing it at a CNAME is invalid and some senders will refuse it.
A domain with no MX is not automatically mail-free: senders may fall back to the A record. If a domain should never receive mail, publish a null MX (0 .) instead of leaving it out.
MX alone says nothing about who may send. That is SPF, DKIM and DMARC — and they belong in the same zone.