[Exim] An idea re stopping spam

This may have been thought of before, but...

If spam filtering (not just RBL, but rules-based filtering) were built
into Exim, *and a time penalty were introduced on receipt of a spam*, ie
the connection was held open but Exim failed to respond for a long period,
might this, if widely used, make it harder for spammers to operate?

Would it be possible to implement this in exim.conf using the current
version of exim?