When you talked about me being able to prove that it is her and not me (or you) Wouldn't the same thing stand to reason with a bunch of emails that I have sent to her and my ex that I know will be used against me, but dont want to? What's good for the goose... ?
It's easier to prove the origins of email than it is to prove the origins of message board posts.

When you send e-mail, it passes through a number of servers to get to its destination. The origin is recorded, as is the entire path the email takes. Some email clients even include the name of the originating computer in the headers, which makes it awfully difficult to claim that your email account was hacked and someone else sent those emails. You can trace email back to its provider all by yourself, and get a court order to get the rest of the way.

Message boards, OTOH, are accessible from everywhere, including through proxy servers, and are much more difficult to trace without cooperation from both the board's administrators and the board's hosting service before you can even think about trying to get back to its ultimate origin.

Shorter version: It's easier to fake board posts than emails.