I see what you mean here. I've done it in the past, and in my early games I thought I was being clever by doctoring or even faking entire emails from other players - this was on some of the USAK/other name stlye judges where you were unlikely to play the same people again and thus gain a reputation.
I stopped forwarding messages when I realised that if I forwarded something to someone, they would more than likely assume I was doing the same with their mails. I think that was in my first game on this site. Since then I've been clean. Clearly you are going to pass on information occasionally, but being trustworthy with information is a much better reputation to have than the opposite.