How should I configure my hostname/FQDN for a multi-purpose VPS?
Solution 1:
Name it whatever you want. There's nothing magical about mail.example.org; it's just a commonly used name for mail server. The mailserver may as well be named example.org or zaphod.beeblebrox.is.alien.example.org. Heck, it need not be in the same domain as the domain it's handling mail for.