Virtual domains vs alias

Question

Awhile back I was posting quite a bit about mac mail and Windows AD questions. That was in prepartion to move a client from Powweb hosted pop/exchange to they're own Mail server. 
I'm converting them this week but there was something I wanted to make sure could happen.
They essentially have 3 domains.  Domain1 is the primary. Domain2 and Domain3 are for other purposes but all recieve mail most of which is forwarded.
Most of domain2 are just forwards to accounts on the primary. So that's just adding the [email protected] to the user's account?  Does the gui create the alias in postfix automatically?
Domain3 however are not forwards. They are accounts for the owner's family domain. I'm guessing the best thing is to just create a seperate user account for those accounts and keep them seperate?
To allow this to be setup seperately on clients and devices (iphone, android, etc) should i just create a seperate accounts? user1 for [email protected] and user1a [email protected]? If there is a better way to accomplish besides a second user account I'm all for that.
My next question; users that have addresses for domain1 and domain2 listed on their account have the ability to respond with either address? That's using an email client like outlook or thunderbird? This isn't really a necessity as domain2 email addresses are just forwards in powweb to domain1 addresses. I was curious.
User1
[email protected]
[email protected]

Answer 1

If you have the email service turned on, creating a user automatically creates an email account under the domain that was setup for email.  Apple does have virtual domains, both web and email.  I've only worked with web virtual domains and that was very little.  Basically to get two websites running. 
I've never worked with the email virtual domain so I'm not sure how it works.  Email virtual domains are setup in the server admin app under the advanced tab.  My personal email server is Axigen running on Fedora Core which handles multiple domains (not virtual) which is why I have not looked at virtual email domains on Lion Server.
The WGM>Accounts>selected user>info tab is for Open Directory information to my knowledge.  Not sure how, or even if, that interacts with the email service.