The UNIX-to-UNIX Copy Program (UUCP) lets a system copy files to
    and from other systems running UUCP. UUCP is usually used to copy
    files over a dialup connection.

    To route mail using UUCP, ask your system manager to define the
    general gateway in the SMTP configuration.

    To use SMTP to route mail to a system running UUCP, address the
    mail as follows:

    MAIL> SEND
    To: SMTP%"user_name!uucp_host"

    The following example sends mail to geoffrey at host haldir:

    $ MAIL
    MAIL> SEND
    To:  SMTP%"geoffrey!haldir.of.com"

    Ask your system manager whether you need to specify a gateway
    host in mail addresses when you work on UUCP dialup lines.