[exim] Verification on front end spam servers

How do I deal with this?

Server A is a fromt end spam filtering server running Exim. Lowest MX
records point to Server A. Server A filters spam and viruses and
forwards email to server B where the real email accounts are.

So - if servers in the outside world want to verify an email address -
how would that work? Can exim accept a verify on Server B and pass the
request to Server A ? How does that work and how do you set it up?