It is quite possible to register a subdomain as MX. For example "emailserver.my-domain.com": Subdomain is "emailserver" and the domain (zone) is "my-domain.com". In the example we take the 127.0.0.1 (localhost) as IP address.

Since this is a subdomain, this subdomain must logically first be made known in the domain (zone). So the following entry is necessary for the domain (zone) "my-domain.com" in the name server:

Name: emailserver Type: A Value: 127.0.0.1 

If you want to use now for another domain (e.g. "my-kunde.de") than MX "emailserver.meine-domain.de", then for the domain (zone) "my-kunde.de" the following entry must be set:

Type: MX Value: emailserver.my-domain.de.
(There is NO entry for name!)

Then the MX of the domain "mein-kunde.de" points to "emailserver.meine-domain.de" and this subdomain points to the IP 127.0.0.1 again.

You can then use this subdomain "emailserver.my-domain.com" for all your domains at Checkdomain.com as "MX-Record" and only need to change the IP number of the subdomain if you change to another email server.