Protonmail is one of the best, most impervious, most impenetrable encrypted email services on the planet. If you’re interested in descriptive background info about it, you can see the article “The Only Email System The NSA Can’t Access” from Forbes at this link. It was also published by The Sovereign Investor at this link. See also Mark Nestmann’s “Your Data Isn’t Safe in the USA” here. He too recommends Protonmail.
As you may know, with services like that, the emails you send from it are only non-interceptable if you send them to another Protonmail address. Likewise, the emails you receive at your Protonmail address are only non-interceptable if you receive them from another Protonmail address. If you only correspond from your Protonmail address to other Protonmail addresses, no party anywhere can intercept or understand the content of your emails.
There is no fee or cost for the basic service, although there is a premium upgrade available. The link to sign up is https://protonmail.ch/invite.