[exim] system filter error

I have the following system filter:

if $header_subject: contains "tester"
then
save /usr/exim/test/file
endif

When I receive an email with 'tester', I get the following error:
2004-11-17 15:39:40 1CUQ27-000854-Up User 0 set for address_file
transport is on the never_users list
2004-11-17 15:39:40 1CUQ27-000854-Up == /usr/exim/test/file
<system-filter> T=address_file defer (-29): User 0 set for address_file
transport is on the never_users list

Can someone pls tell me what I am doing wrong.