Manually Configuring the Receipt of Internet Mail
Manually configuring Exchange to receive Internet mail involves:
Creating the correct recipient policies, so that your Exchange server receives mail for all e-mail domains that are used by your company.
Configuring inbound SMTP virtual server settings to allow anonymous access, so that other SMTP servers can connect and send mail to your SMTP virtual server.
Verifying that the correct MX records exist in DNS, so that other servers on the Internet can locate your server to deliver mail.