[exim] system filter problems with return_path

Hi,

can someone tell me if its possible to exclude emails in a system
filter using "$return_path" is not someaddress@???. I did try
and google this to get the correct syntax but counldn't find a similar
example, so the syntax I made up is as follows:

if $sender_address_domain is mydomain.net and $return_path is not
alerts-bounces@???
then
unseen save /var/maillog/.${tr{$sender_address}{.}{_}}/
endif

This is successfully saving mails based on the sender_address_domain,
but it continues to save mails even when they have the specified
return_path. Any suggestions on whats wrong?

thanks in advance, Andy.