My first startup tried to answer this. Basically you take a secure hash of every email and store only the hash with a third party (which records the date, that's the monetization) -- but it's really really hard to sell companies on preventative technical solutions that involve math.

It works because you don't need to know which emails/files are important -- storing 250 bytes of hashes is practically free and tells you nothing about the contents, it just authenticates the contents later in court.

A bigger question might be, who sent it? Was it really the person you claim it is?

It's easy to show the receipt of an email and where it came from (especially when you have full headers). But unless the sender bothers to GPG or S/MIME sign the email, in theory, it could have been sent by another person who had access to the account. Or, it could have been spoofed, etc.

Trace of evidence in web email. I just went to Hotmail (an acct I stopped using long ago) and pulled up emails from, yes, 2004. :)

This is why digital signatures were invented.

Here in Australia an opposition leader was undone by a faked email (http://en.wikipedia.org/wiki/OzCar_affair).

RTFA

"Facebook almost certainly has a forensic analysis of Mark Zuckerberg's hard drives and email boxes from this period, because these drives would have been the same ones analyzed in the Winklevoss lawsuit," writes Blodget. "Perhaps the drives show different versions of the emails in question--or no emails at all."