[Exim] Exim Filter bypass problems

Could somebody please tell me how to stop exim bypassing the system filter
when an e-mail is received from <>.
I can't find anything in the docs concerning this effect, but it seems to me
(which doesn't mean much) that it's an issue to do with trusted users.

Any help would be very gratefully received.

Cheers!

Zed

PS. Happy New Year