[exim] ACL nested too deep - possible loop

Been doing some recursion. I'll probably figure a way around this with a
loop counter. However, it might be nice to add a few features to Exim to
deal with recursion. Just like to throw out some ideas.

The limit it 20. Maybe it would be nice to make it settable?

Is there a recursion loop counter that I can read? If not - maybe there
should be?

Thoughts?