By default, you have no mailbox filters unless your system
    administrator has chosen to set them for you. If mailbox
    filtering has been enabled for your account, when you use
    the web-based interface, a mailbox filter is created for you.
    Your mailbox filters are stored in a mailbox filter file. The
    location of that file is site-configurable. For popstore and
    MessageStore accounts, that location is normally in a directory
    that is not directly accessible by non-privileged users, and
    all modifications to your mailbox filters are done using the web
    interface. For VMS MAIL mailboxes, the filter file normally is in
    your login directory, and therefore accessible by you.

    If the mailbox filter file is accessible by you, you may create
    or modify the filter file using any text editor. The mailbox
    filter file is a text file containing commands in the SIEVE
    language with some extensions. See the System Manager's Guide,
    Chapter 16, for information about SIEVE.

                                 WARNING

       If you edit your mailbox filter file manually, you cannot
       use the web interface any longer. The web interface can only
       read filter files it it has written itself. You can use the
       web interface to create an initial mailbox filter file, and
       then edit it manually, but not vice-versa.