Hello,
I am thinking on which email provider to use as an alternative for Gmail.
I don't expect E2EE because I know nobody who uses either Tutanota or Proton Mail personally. I just want an alternative to Gmail, where I can message people I know who use Gmail securely.
EDIT: I have chosen my email provider as Proton Mail because I can send encrypted emails not only to people using Proton Mail, but to people who don't too, which I feel is better.
If the recipient of my mail uses a OpenPGP-compatible client (say, Apple Mail or Thunderbird), then they should be able to receive encrypted mails if they set it up correctly.
True.
Then with Proton (idk about tutanota) you should be able to mail E2EE with them, since you can import their Public Keys into Proton. It's not just "E2EE Proton2Proton", since Proton uses PGP, you can safely mail E2EE with anyone using PGP.