'I should strees that there is nothing whatever unethical about most of these ploys. There is but one golden rule: ‘Thou shalt not attempt to deceive the GM.’ In one British zine this constitutes the complete house rules. To forge orders from another player, for instance, is absolutely taboo, and would normally result in your expulsion from the game. Quite right too — the GM has enough to do, and should not be required to master the study of handwriting, recognize all his players’ voices on the phone, and so on. There is plenty of scope for evil-doing without this, believe me. (In fact one of the more unorthodox British GMs, Will Haven, did once start a game or two in which even deception of the GM was allowed — indeed, the GM reserved the right to fool around with the play himself! So far as I know, these games were less interesting than average, not more.) So plot away as much as you like, but remember the golden rule; and if you are in any doubt, check first with your GM to see whether he finds your ruse permissible or not.' -Richard Sharp, the game of dplomacy
In this case the GameMaster is pretty hard to fool, but hacking the site to alter messages/orders would be considered a violation of the site rules, and sits nicely with the spirit of diplomacy as espoused in this book.
That would similarly go along with sending emails from a spoofed email address pretending to be another player. (though this may be fraud according to your local legal system, I think most laws required some monetary benefit to come from the deception, and i don't think winning at diplomacy would count for such a purpose)
Again, I wouldn't bother putting the effort into such a scheme because I know it would be pretty damn transparent.