The problem is that there isn't any standard that defines what can and can't go inside the body of an email message. So if you want to post each email message exactly in a thread exactly as is, i.e. each with completely different typography and with all the replies attached and not sanitized in any way, then that's relatively easy. But it's also completely unreadable for more than about 30 seconds, and doesn't allow for good search functionality. These problems aren't a deal breaker if you're only trying to make sense of your own inbox, but when you're looking for specific information across millions of people's inboxes then they're a complete nonstarter.