Tutanota is doing a great job in offering an encrypted email service and an encrypted calendar. The flatpak support has improved a lot over time.
It's awesome but as mentioned in the last review this is an older version and didn't work for me at all so I'll have to follow the instructions from minty_guy. I'll also reach out to Tutanota to see if they can get Mint the current version.
HOWEVER! Mint Software Manager offers much older version, over flathub (3.50.10 as of this writing). For this reason I have looked around and have learned about yet another universal software format for Linux, besides snaps, namely: AppImage. Here I have learned how simply it works: https://www.youtube.com/watch?v=nzZ6Ikc7juw , (also: https://appimage.org) and here I have downloaded the most current version of tutanota client: https://tutanota.com/blog/posts/desktop-clients/ , 3.66.7
Tutanota is a really nice email client with a privacy and security oriented approach. I have chosen it over Proton Mail and other similar services to gradually get completely out of the Google, Yahoo, and Outlook ecosystem. I have high hopes from this email client, and I am positively impressed by their approach to email. I hope this service eventually becomes the first choice of people all over the world. (The only problem now is that the software installed through software manager on Linux Mint is an old version which can not be updated through the update manager.)