1) A rental car, like a rented house, is still under your control. But your e-mail account on Google's servers is under their control. They can do whatever they want with it. It's more like your friend letting you use part of his garage to store stuff--a third party still retains full control over the space.
2) As far as I can tell, Google can access your e-mail whenever it wants, so the "locked container" analogy also fails.
Again, I think the fact that Google/Microsoft/etc can scan your emails and documents to send you targeted ads is determinative here. If you're voluntarily exposing the contents of your documents to that process, how can you claim to have an expectation of privacy?
The fact that you own the box that email was delivered to and that email never left that box has no bearing in the matter for the IRS. They still claim the right to inspect it, without a warrant.
Google, Twitter, etc, come into play because they are the kinds of communications the IRS is referring to--electronic communications stored on servers controlled wholly by unrelated third parties.
The IRS isn't going to break into your mail server.
People should really, really be running their own mail servers, and I suspect should be providing their own dialtone as well. The latter is certainly much more complicated and technical, but that level of control is a very nice thing to have.
Google, Twitter, Facebook, etc., don't enter into the situation.
How you transmitted or handled something at a point in time is not relevant to it's status at rest.
This issue here is that the government asserts that email is a communications system only. The problem is that while it does enable people to communicate, it also serves as a filing system to many email users. The law as written in 1986 didn't forsee 25GB mailboxes on O365 or Google with the equivalent of many file cabinets worth of memos, etc.