On the letter analogy, things owned by the government are treated differently from things owned by a 3rd party in the private sector. The government generally is more restricted in its actions. And in the case of the USPS, there are further statutory restrictions beyond those required by the Constitution that are written into law, specifically about protecting the privacy of mail in transit. The same is true with phone calls: wiretapping statutes place certain requirements on when a wiretap can be authorized, beyond the minimums the Constitution would require. One of those (which the AT&T wiretapping scandal hit) is to actually make it criminal for a phone company to volunteer information to the federal government without a warrant, which basically closes down the 3rd-party-consent end-run around search warrants, since it explicitly makes it criminal for them to consent. I do think it's a good idea to consider extending some of these to email, given its pervasive role that's largely replacing what physical mail and phones used to carry. But that requires admitting that email to some extent needs to be treated less like a purely private-sector business, and more like a utility.

Your last point is an interesting one, though. If a provider specifically includes in their service policies a promise not to give up the user's data, then it seems like the provider would have a good claim when they attempt to quash subpoenas, even if the user themselves doesn't have any particular rights in the matter.