PROBLEM:
When sending “Encrypted Emails to Non-Proton Recipients” … it is cumbersome to have to “manually manage” the “Shared Secrets / Passwords” for each recipient … in particular if you are communicating with multiple recipients repeatedly.

In addition … it requires communicating the password through a secure encrypted channel … normally I use “Signal”.

FEATURE REQUEST:
Add an additional field in “ProtonPass” for documenting email recipients and their shared secret for encrypted communications within ProtonMail. You could integrate ProtonPass with ProtonMail such that it detects that you are sending an encrypted email to a Non-ProtonMail recipient … and prompts you to save the recipient and the shared secret in ProtonPass for future subsequent communications with the same recipient. Then when configuring a new email with the same recipient … it provides you with the saved shared secret / password … and auto-populates the required field in ProtonMail when prompted.

Next … to facilitate seamless transmission of the shared secret to the recipient … collaborate with “Signal” … such that you could bundle Signal with a Proton Unlimited subscription. Then when you are sending encrypted emails … and you save the shared secret for encryption and decryption of the message … you could click a “Launch Signal” button from within ProtonMail … that would bring up Signal to auto populate and send the shared secret to the recipient … if they have Signal. If you had to you could store the recipients phone number in ProtonPass along with their email address … so you could key off the email address … then bring up the appropriate Signal contact with the associated recipients phone number as a reference to locate them in Signal contacts … as Signal is “phone number based”.

Signal is a non-profit organization. But personally … I'd be willing to pay let's say an additional $5 per year for the Proton Unlimited subscription …if I knew it was donated to Signal. You could abstract the customer from that donation … and just work it with Signal in the background. Or let your customers know that they are doing a good deed by informing them that $5 of their subscription is going to Signal in compensation for integrating their service with Proton Unlimited.