When sending/reading an email show if the email was sent/received securely
Gmail has this feature where we can see if it's secure or not.
The Newton email client has the "read notification" feature and it works well, but it's $49 a year! https://newtonhq.com/
Being able to verify "delivered", "displayed", or both is essential when dealing with important messages. I don't know the complicated code required to accomplish this, but other insecure email accounts do provide this feature. It would be very much appreciated to see it on the "upcoming" features being considered.
I remember decades ago in AOL where the sender had the ability to delete a sent email to another AOL user if it hadn't been opened by the receiver yet. Also, the sender got feedback when the receiver opened the mail.
The option to do that between ProtonMail users (in "settings" or attached to the entry in the contact list) would be good.
It is a MUST as it can determine the delete time /save time of emails.
Read receipt would be a great feature to add.