Excahnge 2007 Address Policy Help needed….

O.k. so here is my situation I have one domain lets say x.com and I have different company email addresss under this domian. Let’s say @x.com and @y.com. There are some people that are just @x.com and some that are both. I cannot have the default assign both because then it will give they x.com since it is the default the reply from and this will get confusing because they are x.com emails they know them as y.com emails… How do I make it so that when I create a users mailbox that I can choose if they are x.com or x.com & y.com but y.com is the default address. Does this make sense to anyone. I know there has to be a way to do this but I am searching and not finding any instructions on how to get this to work…

TIA!

Jenn