How to set up Postfix to receive mail from two domains?

Hi,

apologies if this is a simple setup problem, but I'm going round in
circles, and haven't yet found something that helps me answer this
problem. Rather than explain my setup, which may be nonsense, let me
ask how you would do this. My requirement is simple. I have a server
with IP address 1.2.3.4, and I want it to receive mail for two
Internet domains.

I have set up an MX record for domainA.com with IP address of 1.2.3.4
I have set up an MX record for domainB.com with IP address of 1.2.3.4

In other words, the server with address 1.2.3.4 is to receive mail for
domainA.com and mail for domainB.com. The server has also been put in
the domain domainA.com. (Although this is probably less important, and
if it is better for it not to be in any of these domains I can remove
it.)

How do I set it up so I can send mail to user1@domainA.com and it gets
sent to the real user userA that exists on the server, and similarly
for user2@domainB.com, this gets sent to userB? (Ie I can ``su -
userA'', and then use ``mail'' to read mail for user1@domainA.com,
etc.)

Any help would be appreciated.