Evidence belongs where it answers an objection the reader is already having, and nowhere else.
A named company similar to theirs does more than a count. A number does more than an adjective. And it works best in half a sentence, dropped in passing, because proof that arrives as a boast reads as a pitch.
If the proof has to be in the subject line, there is nothing else in the email.
How we do it
We do not report open rates, reply rates or pipeline generated. Two of those can be made to look like anything. We report meetings held, because it is the only number we are paid on.