Can they access mail without a warrant that is older than 180 days? If not how the hell is that e-mail law constitutional?
The crux of the matter is "knowing exposure to third parties." So you have an expectation of privacy in sealed postal mail, but not say post cards or anything printed on the outside of envelopes. While a postal service as a matter of course can't read what's in peoples' sealed letter mail, e-mail is sent in plain text and can be seen by any intermediary SMTP server as well as system administrators of the sending and target mail servers. That's just the nature of the protocol.
Moreover, the fact that the U.S. Postal Service is an agency of the government puts it under heightened scrutiny as compared to e-mail providers who are private parties. The fact that people would be horrified if the U.S. Postal Service were tearing open envelopes and scanning mail to send targeted catalogs, etc, to people but accept Google, etc, doing the same thing as a matter of course really cuts at the knees of the argument that people have the same "expectation of privacy" in their e-mail as they do their letter mail.
"Reasonable expectation of privacy" doesn't have an implicit "only as against the government." If you're knowingly exposing the contents to Google and Microsoft to scan, you can't claim to have a "reasonable expectation of privacy" in the contents.
I've stopped putting my public key on emails I send, because almost no one ever encrypted email to me, unless they were specifically sending something perceived to be sensitive.
The ad-bots at Google aren't known to be very gossipy.
(I don't mean this to be snide. I think this is an instance of a very interesting phenomenon, where we reason about our society as if it were a village.)