[exim] Verification Question - Random

Ok - this random command has me interested. I'm using callouts but
trying to reduce them as much as possible. If a do a random call and it
succeeds then the caching with prevent other callouts from doing real
calls. I suppose what I want to do is if the random callout fails then I
should not do a random callout for that domain again for a period of time.

Is there any caching so that if I do a random callout that fails that
the fact that it failed is cached?