A mail system that belongs to a different company than your own and that might also be of a different type. An example could be a public X.400 messaging system in Europe or the Internet’s Simple Mail Transfer Protocol (SMTP) mail system. The term “foreign mail system” usually refers to a mail system other than Microsoft Exchange Server. You can establish connectivity between an Exchange organization and a foreign mail system by installing and configuring an appropriate connector on an Exchange server in your organization.
Examples include: