Mail reception
An MX record specifies a mail server name and priority. Lower values are preferred. The destination is a hostname, not an IP address. That hostname needs working address resolution. Multiple servers may provide alternatives, but DNS alone cannot prove their SMTP availability.
Missing MX and Null MX
Without MX, SMTP can use the domain itself as an implicit destination if it has a usable address. Missing MX therefore does not establish that receiving mail is impossible. Null MX explicitly declares no reception: a single MX record with priority 0 and target “.”. It must not be combined with ordinary MX records.
Reading the result
A Null MX warning can describe an intentional configuration. Check whether your domain should receive mail before changing it. The Email Domain Check displays priorities and checks target resolution; connection tests and delivery are separate tasks.
Working through a finding
Compare the reported server names with your mail provider’s instructions. Use the linked DNS lookup to inspect the destination and the blacklist check to inspect an appropriate public IPv4 address. A blacklist finding concerns reputation; it does not establish whether the server accepts mail for your domain. Receiving and sending infrastructure can be different.