[exim] MDaemon and dynamic screening

I have a client that I am relaying mail for. They are running an MDaemon
9.05 server with dynamic screening turned on. What this does is after 3
unknown recipients it sends a 550 error to the sending host which is my
exim box. Then for the next 10 minutes, whenever my exim box connects to
the MDaemon server it gets sent a 550 error before the SMTP banner comes
up.

My exim box then bounces that email because it has been given a 5xx error
message. What I want to know is if my exim box is behaving correctly. I
assume that it is because 5xx messages are permanent and 4xx are
temporary. I have turned off dynamic screening and this problem is not
occurring anymore, but I now have to explain to management types why the
problem happened in the first place and I want to cover my back that exim
is not at fault.

Regards

-John