[exim] Question about forward recipient verify callouts

I have an unusual situation. I have a customer that I'm spam filtering
for. Email from the net comes to my servers first, I clean it, and then
pass it on.

Their server however for some strange reason returns a 450 instead of
550 for non-existent users. Text is "User unknown in local recipient
table". I'm trying to figure out how to capture and test that response
so I can properly reject this mail rather that retrying for a long time.

Thanks in advance.