The SUBSCRIBE command adds either your address or a specified
    address to the specified mailing list. A response message
    reporting the success or failure of the subscription request will
    be returned. If the file PMDF_MAILSERV_MAIL_DIR:list-name.TXT
    exists, it will be sent to you.

    Use the UNSUBSCRIBE command to subsequently unsubscribe from a
    mailing list; use the DIRECTORY/LIST or LISTS command to obtain
    information on available mailing lists.

    Note that some mail servers can impose restrictions as to who may
    or may not subscribe to a given list.