Exhange Will NOT Pick Mails from ISP

I just set up Exchange 5.5 and connects to ISP periodically. However Exchange will not pick mails? ISP is using Unix and have ETRN setup to fwd mails to machine requesting, as my exchange server does not have a static IP. I have also set up ETRN butexchange still will not pick the mails. Transfer mode is Inbound & Outbound and routing tables are to my domain name. My ISP hosts my domain. When I use any other pop email programs, I am able receive mails but not with Exchange.
Any ideas??????