rmail-message-filter is a variable defined in `rmail.el'.
Its value is
nil
This variable is obsolete since 23.1;
it is not used (try `rmail-show-message-hook').
Documentation:
If non-nil, a filter function for new messages in RMAIL.
Called with region narrowed to the message, including headers,
before obeying `rmail-ignored-headers'.
You can customize this variable.