I am on the hiring committee at a university mathematics department. As such, I read a bunch of rec letters. All of them are positive and praise the applicants. But, some of them say things like "Candidate X ingeniously solved Problem Y which I, along with Famous People A, B, and C, had no idea how to do." And others just say "Candidate X is diligent, cheerful, and a hard worker."

In short, +1 to the parent comment. It should definitely be possible to gain useful information from an e-mail that focuses on the positives.