pedro, you asked about the strong feelings associated with message-passing, and you got the answer, "It varies". Some players hate it, and some are indifferent to the tactic.
As for it being "close to illegal", that is somewhat like being "kind of pregnant": it's a case of either/or. There are very few forbiddens in Diplomacy -- deception of the GM being pretty much universal -- and in all my games I've never come across one in which message-passing was outlawed. Players can react to the message-forwarding as they choose, but there is no legal recourse within the game for such action.