How to configure mailoffline in Exchange

I have installed a MS Exchange 2000 server but it is pity that I don’t have leased line (I use dynamic address to connect internet). Could anyone consult me how I should configure my exchange server to get and send emails from outsite? is it possible if I register an email hosting with a provider and I configure my email server to be relay agent. Pls kindly advice me any solution for this problem.
Thank you for kind help.
Lam Dat Quan