I have accounts at Protonmail and Tutanota. I like Tutanota better, but the only way to access it is via their app or website. You can't use a third party. There are ways of using a third party app for Protonmail, but it also isn't the most ideal way. Nice thing about Protonmail is that it is PGP, which is compatible with other PGP email services. Tutanota is probably better encryption (subject lines are encrypted too), but less compatible unless you are communicating solely with other Tutanota users.

Both services allow you to send an encrypted email that is password protected to anyone. They click on the link and type in the password you gave them. Tutanota's implementation of this is much nicer IMO as it allows both parties to see a thread of the conversation.

Mailfence is around PGP implementation that is free to try out.

The email service that I use the most actually is Fastmail. I know it gets a bad rep for an encrypted service since PGP is offered directly. But, since encrypted services are only good if you are only emailing other encrypted (and compatible) services, most of the emails I'd send with Proton or Tutanota anyway would end up on someone's Gmail account.

If I really need to send a PGP encrypted message I just use a separate GPG program and paste it into a message that I send on Fastmail. Fastmail has a blog post about this here: https://www.fastmail.com/blog/why-we-dont-offer-pgp/

Main cool feature of Fastmail is how you can create on-the-fly aliases. Let's same my email domain is example.com.

I'm on the phone with a landscaping service and they ask for an email address. The name of the company is Green Landscaping so I tell them that my email is greenlandscaping@example.com. They can send an email to that without me having to do anything and I can reply back using that email or send an email from that address as well. So rather than organizing emails by sender, I've started filtering to folders based on the address that the incoming email is being sent to. I've switched away from Gmail over 2 years ago and haven't looked back. It has been so refreshing.

But, if you are emailing friends or family and can convince them to use an end-to-end encrypted service, then Proton, Tutanota, or Mailfence would be your best bet! Posteo would be a good one too, but I don't have as much experience with that one.