[exim] error in your filter file

Hello,

I am trying to set up a simple filter in a .forward file:

# Exim filter
if $header_from: contains "@domain.tld"
then
deliver SOME_EMAIL_ADDRESS
endif

Now, when I send myself an email from domain.tld, exim log tells me:

R=userforward defer (-17): error in filter file: filtering not enabled

Does this error mean I have an error in my .forward file (if so then what
may it be?) or does it mean that I have not yet enabled filtering? If the
latter is the case, does it mean I need some special user_forward/filter
sections in exim's conf? From reading the docs I had the impression it is
enough to create a .forward file. It is quite probable I have overlooked
something, though.

Many thanks for your advice!

--
Zbigniew Szalbot