After switching to ProtonMail for various reasons I finally got the ProtonMail Bridge set up on my Mac. The web app is really fine and I could happily live with that, but I still love to have all my email in MailMate. I don’t know what took me so long… it’s so easy to set up and works great!
I already had PGP set up as well, but unfortunately encrypting mails doesn’t work that way with ProtonMail. I can still encrypt messages by importing the public keys of recipients in the web app, or having my contacts send me emails with their PGP signature. If ProtonMail knows the contact’s public key, it will encrypt it (there might be some setting necessary in the ProtonMail preferences). I tried downloading my private key so I could directly handle signing and encryption in MailMate but that doesn’t work and the email cannot be sent.
I also set up ProtonMail that all my emails will be signed, so I don’t even have to take care of that. My signature will always automatically be added to my mails.
I finally got a key argument sorted out that had me move away from Tutanota. The use of standardized PGP technology and the possibility to easily use any eMail client I want to send and receive emails is really a key factor for me.