I can confirm the problem is in Mountain Lion's Mail. I have problems with my Exchange account on my MacBook Air which is running on Mountain Lion (10.8), but Mail on my MacBook Pro that's still running on Lion (10.7) works perfectly as expected! Just like it did on Snow Leopard (10.6).
It's not incomplete information: it's just pointing the finger in the other direction for no good reason. Or is it: the others have inventend such a lame product which we now finally implemented according to their specifications and therefore it is now their responsibility to fix our just released product?