Just give users the option to save a sent mail or not (have a box or something that can be checked/ticked next to the send button). It would be handy to have a standard settings as well.

This has already been suggested...
here: https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/42828042-option-to-not-automaticallly-save-all-sent-emails
and here: https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/9034192-allow-the-user-to-choose-whether-an-e-mail-is-sent

Those ideas received almost no attention. I want to try again because I think this would be an easy addition for developers to integrate.