Exchange 5.5 & individual company IDs on a dialup ISP

Hi Netgurus,
Ours is a small org.Currently we have registered our our company domain(companyname.com) on a US based server.Previously we were using the ISP mail service([email protected]) and we are connecting to our local ISP using dial up connection(ie: we don't have a static IP).
After installing Exchange server 5.5 we are able to communicate effectively inside our org(ie https://discussions.virtualdr.com/n LAN).
Today i have downloaded EXCHANGE POP3 and installed it.Now we are able to receive mail sent to individual users at the address [email protected] we are not able to send mails to other internet users.The exchange pop3 fails To Internet(SMTP) operation.
Any suggestions or help ...Please let us know....

With Rgds
Sumesh