Mail addresses are listed in Mail eXchange (MX) records on the name servers.
if you use NS lookup you can type something like
and an address is returned.
now change the program to look for mail servers
and a list of the mail servers and backup mail servers is shown.
Mail servers have a priority, 5 10 15 30 (5 is highest priority) 30 is last choice of server to deliver mail to if the first server is down.
It's normal that mail is delivered to whatever address is maped to mail.domain.com, however I've noticed that a lot of spam checking software will reverse lookup domain names based on the top level only, without bothering to check the MX records, so mail will be bounced despite the fact it is legitimate.
(this won't bother you unles your mail server pointer is to another address to your top level domain pointer.)