Exchg2000 cannot sent mail to internet

Recently I setup an MS Exchange2000 email environment for my customer customer.com,
they already register their domain to INTERNIC
as company.com ,which handling by their ISP.

They using a 3rd party DomainPoP Connector
to retrieve all their domain mail by ADSL dail-up ( logon as abc@company.com ), it tested OK.

Now, user (abc@company.com) cannot sent mail to internet througt exchange2000. What should I do in SMTP Virtual Server so that It can sending mail to internet?
Fyi, Exg2000 also a Domain Controller with Highest DNS Level as root (.) .