All traffic between Hub Transport servers is encrypted by using TLS with self-signed certificates that are installed by default by Exchange 2007 Setup.
All traffic between Edge Transport servers and Hub Transport servers is authenticated and encrypted. The underlying mechanism for authentication and encryption is mutual TLS. Instead of using X.509 validation, Exchange 2007 uses direct trust to authenticate the certificates. Direct trust means that the presence of the certificate in Active Directory or ADAM validates the certificate. Active Directory is considered a trusted storage mechanism. When direct trust is used, it doesn't matter if the certificate is self-signed or signed by a certification authority. When you subscribe an Edge Transport server to the Exchange organization, the Edge Subscription publishes the Edge Transport server certificate in Active Directory for the Hub Transport servers to validate. The Microsoft Exchange EdgeSync service updates ADAM with the set of Hub Transport server certificates for the Edge Transport server to validate.
By default, traffic between Edge Transport servers in different organizations is encrypted. By default, Exchange 2007 Setup creates a self-signed certificate and TLS is enabled. This allows any sending system to encrypt the inbound SMTP session to Microsoft Exchange. Also by default, Exchange 2007 tries TLS for all remote connections.
Authentication methods for traffic between Hub Transport servers and Mailbox servers differ when the Hub Transport server roles and Mailbox server roles are located on the same computer. When mail submission is local, Kerberos authentication is used. When mail submission is remote, NTLM authentication is used.
Exchange 2007 also supports Domain Security. Domain Security refers to the set of functionality in Exchange 2007 and Outlook 2007 that provides a low-cost alternative to S/MIME or other message-level over-the-Internet, security solutions. The purpose of the Domain Security feature set is to provide administrators a way to manage secured message paths between domains over the Internet. After these secured message paths are configured, messages that have successfully traveled over the secured path from an authenticated sender are displayed to users as "Domain Secured" in the Outlook and Outlook Web Access interface. For more information, see Planning for Domain Security [ http://technet.microsoft.com/en-us/library/bb124392.aspx ] .
Many agents may run on the Hub Transport servers and Edge Transport servers. Generally, the anti-spam agents rely on information that is local to the computer that the agents run on. Therefore, very little communication with remote computers is required. The exception is recipient filtering. This requires calls to either ADAM or Active Directory. It is a best practice to run recipient filtering on the Edge Transport server. In this case, the ADAM directory is on the same computer as the Edge Transport server and no remote communication is required. When recipient filtering has been installed and configured on the Hub Transport server, recipient filtering accesses Active Directory.
The Protocol Analysis agent is used by the Sender Reputation feature in Exchange 2007. This agent also makes various connections to outside proxy servers to determine inbound message paths for suspect connections.
All other anti-spam functionality uses data that is collected, stored, and accessed only on the local computer. Frequently, the data, such as safelist aggregation or recipient data for recipient filtering, is pushed to the local ADAM directory by using the Microsoft Exchange EdgeSync service.
Journaling and message classification run on Hub Transport servers and rely on Active Directory data to function.