Filters
Filters are a generic mechanism for automatically adding or removing Tags to messages that have just been discovered.
Some of the Mailpile default behaviors (flagging messages unread or
placing in the inbox) are implemented using filters that are configured
by the --setup command.
FIXME: Describe filters in more detail.
This document needs to be fleshed out. In the meantime, please see
help filter in the CLI, or check out mailpile/plugins/filters.py in
the source.