What is the DNS Validation Error (Code: 1004) Invalid content for an MX record?

Question

cloudflare

Hello.
For the first time I meet the error DNS Validation Error (Code: 1004) Invalid content for an MX record
When I tried to set the MX record, I tried to specify the IP and also the domain, it still does not work.
Tell me what could be the problem?

Answer 1

tried specifying ip and domain

domain is necessary, and priority