Outside emails on my Microsoft Exchange server 2007

Hi there, i am running a microsoft exchange server 2007 and for over a month i cant figure out how to send and receive outside emails on my exchange server so that all of the mailbox users can have their emails from the web based accounts that they already have. It is a typical install without edge transport role on it and we have dynamic ip address so pls if any one knows how to it will be gr8 help you can send me an email on nedelkovskiv@yahoo.com or you can just post the solution here thx in advance.