The domain name in the email address is fairly irrelevant. The name used in the MX record doesn't really matter either. ISPs will often use something like mail.customer-domain.com in MX records, all pointing to one server whose real name is obviously not mail.customer-domain.com. What does matter is the servers actual hostname.

When talking via SMTP, your server will identify itself in the SMTP conversation using its full hostname, in this case 'server.example.com'.

The A record for 'server.example.com' should point to the IP of your server, and the PTR record for this IP address should match the hostname.

server.example.com. A 82.197.45.124

124.45.197.82.in-addr.arpa. PTR server.example.com