I know you provide this service but where can I research configuration and activation? I know as the holder of a free DNS service I'll have to pay for this particular service.
you would create a host record for your mail server... like mail.domain.com and point it to your IP address.. example 1.2.3.4... then create an MX record
where mail for zone domain.com is handled by server mail.domain.comortnumber like mail.domain.com:2525