I find it a little puzzling that when it comes to proving emails there isn't more fuss and bother than there appears to be. Doubtless I'm missing things that Slaw readers, particularly the e-discovery folks, perhaps, will point out to me. But it strikes me that the very thing making email so wonderfully convenient is also likely to make it difficult to prove an email in a court of law. I'm referring, of course, to email's etherial nature: the fact that an email exists both everywhere and nowhere in particular.
Unlike a printed letter, which has an actual, physical, unique existence, an email is always a copy, always a fresh coming-into-being, essentially the predictable result of a set of digitized instructions for a computer; and this set of genetic instructions, as it were, can and usually does exist on any number of computers.
Of course, the sending of an email leaves a trail of named routers (see this example, if you're not familiar with what lies behind your "To:" and "From:" boxes) that can in most cases take you back to the ISP account of the originator of the email. If you have access to that person's computer, you might be able to find a record of its creation. Or it may be that the sender's ISP keeps a copy of all email that passes through its SMTP server. In such a case, I imagine that you might be able to get a file that represents the email in its first version. I don't know how easy this search for the first copy would be in practice, or whether that is the preferred route to go when seeking to prove the contents of an email.
In addition to the problem of tracing an early, or the earliest, copy, there's the matter of forgery or, as we might say nowadays, redaction. It is an easy thing for anyone in the chain of recipients to alter the content of the email. Again, this will likely leave some electronic traces that might be discoverable if one had the time and the ability to explore the hard drive of the redacteur, though I'm uncertain about how easy or reliable such an investigation would be.
It would seem, however, from the relative lack of reported cases on the subject, that it rarely happens that there is a dispute about the accuracy of the content of an email. Where there is no dispute, proof is easy, of course: simply a matter of both sides accepting the content as accurate.
Even so, if I were in a business that was in any way crucially dependent on email, I'd be nervous about being able to prove their content down the line. In which case, I might be interested in looking at enterprises that register emails. I've found two, RPost and ReadNotify. RPost offers "Registered E-mail" that offers "Verifiable proof of: Sending and Delivery – Content of e-mails and attachments – Uniform time sent and received." To see more, look at their page entitled "Anatomy of a Registered Receipt.". ReadNotify seems to offer the same or similar functionality, and they, too, have a typical receipt you can examine to get a better sense of what they provide.
As I said earlier, I'm interested to hear what those who deal in proof of emails regularly have to say about how it's done and what the problems in practice are, if any.