MS Exchange configuration.

Hi friends,
I have windows2003 server configured as DC01.company.local.i have installed DNS,DHCP also.working fine.all internal mails are working fine.Now i want register a dns name like abc123.com(assume)with the ISP.my question is what has to be changed in my active directory and exchange so that users can mail out and mail in through internet.coz right now if any user send mail internally it is user@company.local.
Please help.