This may be a very novice question but I'll ask anyway. What permissions do I need to set on an Exchange server please?

The reason I ask is that I've moved mailboxes from a 2003 Exchange server to the new 2010 and now the mailboxes don't receive any mail! Mail still comes in to the 2003 box for one moved mailbox that had errors on the move. I'm guessing it might be permissions, which I haven't done anything with since I first set up the server.