[Exim] filtering (bogofilter)

I'm having trouble finding a way to use bogofilter together with exim's
filtering.

The problem is that bogofilter can return a value that tells if it think
the mail is valid or a spam mail. Or it could add one header to the mail
in passthrough mode.

The return value is useless as exim rules can't check the return value
so I thought that I was smart when I checked for that special header and
if it was not there use the pipe to make it first go to bogofilter then
exim mail it back to me but that changes the sender header that I use to
filter mail from mailing lists.

Any idea on how to solve this.