I have a client that pops mail to Exchange 2K. There are a couple of people not in the office, and we wanted them to pop thier mail from the ISP. Problem is that when the internal people wanted to email them Exchange would gobble the email up. Here is a way to fix that.
I Googled ‘Exchange email address same domain’

http://support.microsoft.com/default.aspx?scid=kb;en-us;319759