DNS? DNS is easy...
DNS should be the last of your worries - it's one of the few things that you point, it works. As long as your DNS records worldwide point to your box, whatever you specify on the DNS settings will be considered gospel. Takes 24-48 hours on average to propagate around, so don't freak if it doesn't work right away but as mentioned if all your DNS points to your DNS namerservers (NS's) on only your box (as in you don't use backup DNS) then what you change should change instantly.
Figure it's the giant Net phone book. If all the directories point only to your phone # (IP, your NS's on the box) then all the calls will only go there. So if you change something where the phone 'rings' and not change the phone # itself - then you're in control and whatever's supposed to happen at your location rules. Like an answering system that answers the call or redirects it, etc. - you make the decisions.
If you have backup DNS, the changes make take longer. If you change the IP's completely, obviously it takes time to update all the 'phonebooks' in the World and therefore 24-48 hours propagation.
That's why I mentioned leave the DNS for the mail.domain.com part be and just redirect the DNS settings for the domain elsewhere. Or, more preferably, just what you did with .htaccess. The script version works the same way, has a backup for browsers that freak out and don't get or can't run the script so people end up in the same place. Many settings in Plesk also have Domain Alias stuff to do this but there's questions as to what the fool mail service then does if you Aliased it to change the HTTP destinations.