Multiple emails accounts(with different from domains) on multiple domains on 1 server
 

Ok, my title is a little messy..maybe...
Anyhow, my question is this. I own a server, with one IP. It hosts multiple domains. All settings are managed by me, I have full root access.


Say I have www.domainA.com and www.domainB.com hosted on the same server.

Each domain has it's own user. Say and .

What I am trying to do is set up the domain email system so when userA sends an email, it comes from userA@domainA.com and userB gets userB@domainB.com

What do I need to set up? I hope this makes sense. I have set up everything fully, this is my only problem now..
Whenever I send an email from any account, it comes from the
user@domainA.com
address, where
domainA.com
is the first domain I set up with my server. I just can't seem to find a way around it.. and google sadly couldn't help either..

Thanks for any help! Other than that I haven't had any problems.