return path not passed to spamassassin

I have had the problem that SPF checking does not work when spamassassin 
is called from exim.  It works fine when running spamc after a message 
has been delivered.  I believe that I have tracked the problem down to 
the return path not being added to the headers by exim until after 
spamassassin is finished.

It seems to me that this is something sa-exim should have configured 
automatically...of course it's just as likely something I should have 
configured but didn't.

I found this thread that discusses the same problem.

http://www.nabble.com/Availability-of-return-path-to-Exiscan-t49803.html

Please provide suggestions as to the best way to solve this.

Thanks.

-- 
Good day, eh.
Chris