I think comparing "almost illegal" to "almost pregnant" is a little silly. Nevertheless, some players consider it near heresy. I do it rarely, but I'll do it in particularly dire circumstances. I've even been known to falsify e-mails from "others" to make my point. Anything for the game, you know.

Some players oppose the tactic so much they don't care what the e-mail says, they will immediately go after you for being so disreputable. I had one player call me "insulting to the hobby," which seemed a bit harsh. So there are risks, but that's always true. The tactic, however, is not (and really cannot be) forbidden.

I'll add if people forward me an e-mail from someone else, I generally disapprove. Not because I'm a hypocrite, but because it isn't for something ridiculous enough. It is, for me, a last ditch effort. Forwarding e-mails to prove a 1901 FE alliance seems a little unnecessary, and while I won't insult the player or throw my forces haphazarly against them for revenge, I will doubt their cunning and probably consider them easy meat for the rest of the game.