Hello Laurent,
I totally share Your opinion regarding gmail from the technical point of view, but nowadays we have to take in account the security much more.
Bearing that in mind You can get the most with an account from mailbox.org.
It is possible to access your mailbox via Tor-Browser and onion-address, encryption in many varieties is implemented and the account can be fully anonymous; payment is possible by cash via letter. You can use Your keys or generate new ones. A test-account for one month is for free. Even though it is a German provider (more trust than an American), the homepage can be switched to English; anyone interested should give it a chance. I am very satisfied now for over a year.
Regards
Andreas