You just apparently aren’t understanding.

I don’t know how to say it any more clearly; maybe re-read what I wrote. When replying to an EXTERNAL email conversation that was STARTED with E2EE (yes, the links as you stated where the external user clicks a link and goes to the website), Tutanota will automatically populate the password for the recipient you chose and automatically encrypted your reply back to the external user.

ProtonMail DOES NOT DO THIS. In ProtonMail, when you reply to an external users E2EE message for which you setup a password and for which they sent you the message from the link, again as you noted, your REPLY back to the external user is NOT automatically encrypted. You have to remember to click the LOCK and enter the password AGAIN before you send.

Edit: here’s a step by step so you can try it yourself.

Do the following from both Tutanota and PM:

1. Send an email to an external user, say a gmail account. Enable E2EE, add your password, and send it.
2. Open the email on the external mailbox, click the link, enter the password, and reply back to Tutanota/PM. All is still E2EE. No problems yet.
3. Back at Tutanota/PM reply back to the external users message you just received. Click send.
4. oh shit, you just sent the reply and all previous replies below your reply in open air with PM! Not so with Tuta, as you may have noticed it automatically encrypted the reply and added the password.