Regexp to match header fields that Rmail should normally hide.
(See also `rmail-nonignored-headers', which overrides this regexp.)
This variable is used for reformatting the message header,
which normally happens once for each message,
when you view the message for the first time in Rmail.
To make a change in this variable take effect
for a message that you have already viewed,
go to that message and type M-x rmail-toggle-header twice.