I'm using countermail.
https://countermail.comWe are using a strong encryption protocol called OpenPGP, with 4096 bits encryption keys to protect your data. To the best of publicly available information, there is no known method which will allow a person or group to break OpenPGP:s encryption by cryptographic or computational means.
This is useless. You are still relying on their word that they are actually encrypting your messages and not saving the original before encrypting it. There is no way you can know they don't do this without doing the encryption yourself.
You can PGP encrypt it on your PC before sending and eliminate this risk entirely - you can do this with ANY mail provider - no need to pay countermail for something you can't be sure they actually do.
Since there are various PGP software providers to choose from (GoAnywhere, Gpg4couwin etc)
Do you have a few you could recommend?
From what I understand those are just frontends, most of these use GnuPG as a backend which is the de-facto standard:
https://gnupg.org/GnuPG is a command-line tool, so if you find it difficult to use you might want to install a frontend too or an email client that has plugins for it like thunderbird.
Frontends:
https://www.gnupg.org/related_software/frontends.htmlthunderbird plugin:
https://addons.mozilla.org/en-US/thunderbird/addon/enigmail/