Question : Email server using Xchange

I just wonder, if I want to use MS Exchange as an e-mail server, then which part (xchange component) that will handle the incoming mails forwarded by my ISP ?
SO far, what I have ever tried is to set my Xchange server as SMTP gateway for sending email out to internet, but I have no idea about how Xchange handles the incoming email, and then distribute the e-mail to respective user's mailbox.
thanks

Answer : Email server using Xchange

Now the message headers in this topic are messed up it only remains to see if you give me or yourself the points!