The mail system can use UUCP (UNIX to UNIX copy) as a transport mechanism for messages from one machine to another. While UUCP itself is configured and documented separately (see Administering network services), the mail system must be explicitly configured to use UUCP, as initially, it is not set up. Enabling (or disabling) UUCP with the Mail Manager is fully covered in ``Mail delivery channels''.
What the UUCP channel does is to treat the /usr/lib/uucp/Systems file (part of the standard UUCP configuration) as a list of machines available to accept UUCP requests. Any mail destined for one of the listed machines will be forwarded to the UUCP system for transport.
The mail system builds a parallel database file (a fast, searchable copy of /usr/lib/uucp/Systems). If you later modify /usr/lib/uucp/systems, you will need to re-run the Mail Manager. When you exit the manager, it will automatically detect that the Systems file has been updated and ask if you wish to rebuild the database file. Answer yes to the query and the configuration is compete.