You can configure your Microsoft Exchange Server 2007 organization with only Hub Transport servers for message routing and delivery or with Hub Transport servers and Edge Transport servers. When you use only Hub Transport servers, these servers are responsible for:
Messaging routing and delivery within the organization.
Receiving messages from outside the organization and delivering them to Mail-box servers within the organization.
Receiving messages from Mailbox servers within the organization and routing them to destinations outside the organization.
When you use Hub Transport and Edge Transport servers, message routing and delivery works like this:
Hub Transport servers handle messaging routing and delivery within the organization.
Edge Transport servers receive messages from outside the organization and route them to Hub Transport servers within the organization that, in turn, deliver them to your Mailbox servers.
Hub Transport servers receive messages from Mailbox servers within the organization and route them to Edge Transport servers, which, in turn, route them to destinations outside the organization.
The primary mail protocol used by Exchange Server 2007 is Simple Mail Transfer Protocol (SMTP). This chapter discusses how transport servers use SMTP for routing and delivery, as well as how you can view and manage transport server configurations.