Evidence is not the same as conclusive evidence, and it would be perfectly proper for one side to introduce emails and the other side to claim that a particular email or emails are forged.  It would then be up to the trier of fact (jury or judge) to decide who is most convincing on that meta-issue.

John Cowan          http://vrici.lojban.org/~cowan        cowan@ccil.org
And now here I was, in a country where a right to say how the country should
be governed was restricted to six persons in each thousand of its population.
For the nine hundred and ninety-four to express dissatisfaction with the
regnant system and propose to change it, would have made the whole six
shudder as one man, it would have been so disloyal, so dishonorable, such
putrid black treason.  --Mark Twain's Connecticut Yankee