[exim] filter on MAIL FROM: in acl_check_rcpt

Hi

Is it possible to filter on the "MAIL FROM:" header in acl_check_rcpt: ?
( I can't find any variable that works :( )
The reason i want to do this is because i would like to ban 'firsttime'
ip's that try to send mail from a address that begins with - or _, i'm
getting alot of those :(, and i would also like to have a more friendly
(Swedish) error message if somebody from my domains try to send a mail
without authentication,