[EXIM] Melissa, Papa & Happy99 Filters

I do not have any previous experience in setting up mail filters with exim.
Please could someone tell me how to go about setting up a global filter
for Melissa, Papa or Happy99 step by step.

When I try "exim -bf" on my mail filter that I attempted to make for the
Melissa virus, I received the following error message:

Filter error: unavailable filtering command "fail" near line 8 of filter file

Any suggestions would be very much appreciated.