If you do not specify an input file, the MAIL$MAILFILE_INFO_FILE
    returns information about the currently open mail file.

    Folder Action Routines

    If you use the item code MAIL$_MAILFILE_FOLDER_ROUTINE to specify
    a folder name routine, MAIL$MAILFILE_INFO_FILE passes control
    to a user-specified routine. For example, the folder action
    routine could display folder names. The user routine must return
    a 32-bit integer code. If the return code indicates success, the
    interaction between the user's routine and the callable routine
    can continue.

    The folder action routine passes a pointer to the descriptor of
    a folder name as well as the user data longword. A descriptor of
    zero length indicates that the MAIL$MAILFILE_INFO_FILE routine
    has displayed all folder names. If you do not specify the item
    code MAIL$_MAILFILE_FOLDER_ROUTINE, MAIL$MAILFILE_INFO_FILE does
    not call any folder action routines.