Individual users can use personal PMDF mailbox filters to prevent delivery of unwanted mail messages to their mailboxes, or to define vacation notices. Or the PMDF manager can create channel level filters or a system wide filter.

Mailbox filters are text files containing commands in the SIEVE language (see Section 16.2.7). Mailbox filter files may be created by hand using any text editor. PMDF also supplies a web-based interface for creating mailbox filters, see Section 16.2.6. System managers can customize this web interface as described in 16.2.6.

Through the web-based interface, users can construct and manage the screening rules applied by their personal mailbox filters and configure their vacation notices, or the PMDF manager can construct an manage the screening rules for channel level filters or a system wide filter. Personal mailbox filters are supported for use with the l (lowercase "L"), msgstore, and popstore channels. Channel level filters are supported for all channels.

The web-based interface allows you to set up eight distinct filters: four to identify messages to always keep, the "Accept filters"; four to identify messages to always throw away, the "Discard filters". The Accept and Discard filters operate on envelope and header source addresses, header destination addresses, and phrases or words appearing in the Subject: header line or body of the message. The eight filters are thus known by the names Accept From, Accept To, Accept Subject, Accept Body, Discard From, Discard To, Discard Subject, and Discard Body.

The web-based interface also allows users to set up vacation notices. See Section 16.2.8. Note that if you set up a vacation notice for a channel level filter or the system filter, it will just be ignored.

Users' personal mailbox filters are applied first. If a personal mailbox filter explicitly accepts or rejects a message, then filter processing for that message copy finishes. But if the recipient user had no mailbox filter---or if the user's mailbox filter did not explicitly apply to the message in question---PMDF next applies the channel level filter. If the channel level filter explicitly accepts or rejects a message, then filter processing for that message finishes. Otherwise, PMDF next applies the system filter file, if there is one.

By default, each user has no mailbox filter. When a user uses the web-based interface, and mailbox filtering is enabled for the channel which delivers mail to their mailbox, a mailbox filter is created for them. Each mailbox filter is stored in a disk file, the location of which is controlled with the filter channel keyword as described in Section 16.2.1.

Channel filters are not present by default. If channel level filtering is enabled for a channel, then the PMDF manager can create a channel level filter using the web-based interface, by authenticating as the "address" @channel-host-name where channel-host-name is the official host name of the channel in question, and providing the password for the PMDF server account.

The PMDF manager can create a system level filter using the web-based interface by authenticating as the "address" @---that is, specifying the single character @ as the "address"---and providing the password for the PMDF server account.