Currently when sending Password-protected emails to non-proton recipients the sender has to select the password, notify the recipient of the password, and manually record the password for future correspondence. It is tedious and has two major issues.
1. The first issue is if you want to send the message to multiple people they must both have the same password.
2. If a recipient receives password-protected emails from more than one protonmail senders, they would need to track and keep a log of passwords for each and every sender.
A solution would be to have the recipient select a password to decrypt the email link and store the password on a server in the same manner that passwords are handled for any other email (proton to proton, etc).
Because the process of sending password-protect emails is tedious, it is rarely used (by me at least) and any email sent or cc'd to a recipient is on the gmail, yahoo, hotmail, etc. servers in plain text where companies like google can glean the content. Furthermore in the US, Section 702 of the FISA act allows the government to gather texts and emails from overseas without a warrant. Protonmail originates from overseas (Switzerland).
An easy way to send and receive secure email from protonmail to non-proton recipients would go a long way to making protonmail secure.