In a typical Exchange Server 2007 scenario, you’ll have at least two physical servers, one of which is dedicated to the Edge Transport role. Here’s how a message moves between the server roles in this scenario. When the Mailbox server notifies the Hub Transport server that the Mailbox server has a message for delivery, after the Hub Transport server processes the message, it decides whether that message should be transferred to another (or same) Mailbox server in a site, transferred to a Hub Transport server in another site, or forwarded to the Edge Transport server. All messages directed to Internet users (or users outside your Exchange organization) are forwarded to the Edge Transport server. This server uses public DNS servers to locate the destination mail server, then transfers the message by using SMTP or simply forwards the message to a smart host. Optionally, the Edge Transport server also applies transport rules on message or address rewriting, if these options are configured.
On the other side, when an inbound message from the Internet reaches the Edge Transport server (this server is reachable via MX records in your domain’s public DNS zone), antispam and antivirus agents process the message, the Edge Transport server applies transport rules (if there are any) to the message, and finally the Edge Transport server transfers the message to the Hub Transport server. The Hub Transport server searches the Global Catalog to locate the recipient’s Mailbox server, then transfers the message to the user’s mailbox.