In order to bloc spam, emails nowadays contain a form of digital signatures that verify their authenticity. This is automatic, it happens on most modern email systems, without users being aware of it.
This means we can indeed validate most of the Wikileaks leaked DNC/Clinton/Podesta emails. There are many ways to do this, but the easiest is to install the popular Thunderbird email app along with the DKIM Verifier addon. Then go to the Wikileaks site and download the raw source of the email https://wikileaks.org/podesta-emails/emailid/2986.
As you see in the screenshot below, the DKIM signature verifies as true.
If somebody doctored the email, such as changing the date, then the signature would not verify. I try this in the email below, changing the date from 2015 to 2016. This causes the signature to fail.
There are some ways to forge DKIM-signed emails, specifically if the sender uses short keys. When short keys are used, hackers can "crack" them, and sign fraudulent emails. This doesn't apply to GMail, which uses strong 2048 bit keys, as demonstrated in the following screenshot. (No, the average person isn't supposed to understand this screen shot, but experts can).
What this means is that the only way this email could've been doctored is if there has been an enormous, nation-state level hack of Google to steal their signing key. It's possible, of course, but extraordinarily improbable. It's conspiracy-theory level thinking. Google GMail has logs of which emails went through its systems -- if there was a nation-state attack able to forge them, Google would know, and they'd be telling us. (For one thing, they'd be forcing password resets on all our accounts).
On the other hand, Wikileaks only shows us some of the emails. We don't see context. We don't see other staffers certain it's going to be somebody else for VP. We don't see related email discusses that cast this one in a different light. So of course whether this (verified) email means they'd firmly chosen Kaine is "mostly unproven". The purpose of this document isn't diagnosing what the emails mean, only the claims by Hillary's people that these emails have been "doctored".
As a side note, I offer a 1-BTC (one bit coin, ~$600 at today's exchange rate) bounty to anybody who can prove me wrong. If you can doctor the above email, then you win the bounty. Some rules apply (i.e. it needs to be a real doctored email, not a trick). I offer this bounty because already people are trying to cast doubt on whether DKIM works, without offering any evidence. Put up or shut up. Lamers aren't welcome.