Welcome to LQ!
AFAIK:
Your MX host needs an A Record.
Code:
dig domain.com MX +short
The MX Record points to an IP.
should point to mail.domain.com
The A Record for domain.com and mail.domain.com can be the same IP.
That IP must have an rDNS assigned to it. (mail.domain.com),
but you can only have one rDNS (reverse DNS) for one IP.
The host assigned that IP, or IPs can have any hostname you choose.
Someone else here will point out my inconsistencies, to be sure. So don't run off and "make it so" with further additional input.