You need to choose the best type of connector to create a link to another Exchange 2000/2003 routing group, Exchange 5.5 site, or foreign mail system.
The general rule of thumb for choosing connectors is simple: use a RGC unless there is a specific reason to use another connector. Luckily, these reasons are easily listed and remembered:
Use the appropriate specific connector when interfacing your Exchange organization with foreign mail systems. Exchange Server 2003 supports specific connectors for Novell GroupWise and Lotus Notes; SP1 provides support for Notes R6. In addition, the Exchange Calendar Connector allows users in your Exchange organization to bidirectionally access calendar data with these systems. Exchange 2000 also provides connectors for cc:Mail and Microsoft Mail. Other vendors may offer additional connectors.
Use the SMTP connector for gateways to the Internet, other generic foreign SMTP mail systems, or Exchange 5.5 sites that are not part of your organization. These Exchange 5.5 sites must have the Internet Mail Service configured.
Use the SMTP connector for connections between routing groups in your organization if you want to take advantage of the additional features of the SMTP connector, such as support for outbound SMTP security. This is desirable if your physical network link with the other site passes through the Internet and not over a private leased line.
In Exchange 5.5 you would use the X.400 connector ...