[exim] Not sure how to block this spam ?

Hi

I am receiving spam to one of my email domains now that does not have my
address in the TO or CC headers which I thought should not work as the
server only delivers to addresses that are configured.

When I look at the headers there is a field called "envelope for" which
contains the address to which it is delivered

I am sure it must be easy to stop this but I am a little lost
which document will best explain what I should do ?

Will this be in ACLs somewhere or the router ?

Kind Regards
Hill