An email is just a text file. It could have been saved for all that time, or written yesterday. In the context of an outstanding accusation of forgery, more easily-forged documents have no evidential value.
The first thing that stands out to me is that the errors in punctuation (conveniently called out with "(sic)") are similar between both authors. For example, using a comma where one would normally end a sentence and omitting apostrophes in contractions and genitive constructions.
A friend of Mark from this time could easily say if this is his writing style or not.
Mark's style could also be forged with a bit of frequency analysis so I'm not sure it proves anything (although it's admittedly unlikely).