Automatically update e-mail addresses based on recipient policy

Ok, I'm a little new to Exchange.   I have EX2003 with Outlook clients.  We are a very small company with only 4 employees. I dont understand the way Exchange "assigns" email addresses.  For example:  I have a user named John Smith.  I created his AD user account and enabled mail for him.  his email address was "john@abc.com".  The problem is that up until now, we've been using POP email and we'd given John the email address "accountspayable@abc.com".  So, I go into ADUC and "add" the "accountspayable@" email address for him.  I select "Set as Primary" for that address.  But after about two minutes, it reverts back to "john@abc.com"!  I realize that this probably has something to do with the "automatically update e mail addresses based on recipient policy" check box.  Help!