You can use Internet Mail Service, the X.400 Connector, or the Site Connector to connect Microsoft Exchange Server sites through the Internet. If mail must pass through one or more SMTP hosts to reach its destination, Internet Mail Service must be used. Note that data is not encrypted when it is sent through these connectors unless messages are encrypted using advanced security.
The Site Connector uses RPCs to communicate between servers. Server-to-server RPC sessions are always encrypted. If you use a packet filter, you must configure it the same as you would when enabling client access over the Internet.