[Exim] altering outgoing mail

Hello Exim-users,

I want my smtpserver (exim...) to filter messages before they are being sent
away. I do it with a system-wide filter, but it doesn't seem to work very
well.

My settings are:
--exim.conf--
system_filter = /etc/exim.filter
system_filter_user = exim
system_filter_pipe_transport = address_pipe

--exim.filter--
# Exim filter
if $message_body_end: contains "beer"
then pipe "/usr/bin/sed 's/beer/water/'"
endif

Messages without beer do arrive, but messages with 'beer' don't.

Error:
--mainlog--

date time messageID <=rmeijboom@??? H=userhost.domain [user ip address]
P=smtp S=625 id=blablabla
date time messageID original recipients ignored (system filter)
date time messageID ** |/usr/bin/sed 's/beer/water/' <system-filter>
T=address_pipe: return message generated
date time messageID <= <> R=messageID U=exim P=local S=2252
date time messageID Completed
date time messageID original recipients ignored (system filter)
date time messageID => |/usr/bin/sed 's/beer/water/' <system-filter>
T=address_pipe
date time messageID Completed

the messages do not arrive

Anybody knows what i'm doing wrong? ok forgive me i'm a total newbie at
exim, installed it today....if this is the wrong way, is there another way
to do it?

Thanx!

Robin Meijboom