[Exim] User-Filter with Error 550

Hi,

can anybody please explain to me how to
install a filter (.forward) that sends
back an SMTP-Error 550 'user unknown'
to a sender if certain conditions apply.

This is probably the easiest to get off
an unwanted list.

Something like that:

# Exim filter
if $reply_address contains "@spam.site" and
$reply_address does not contain "postmaster@"
then
mail "Error 550"
seen finish
endif

Best regards

Klaus