I love the folder system on Protonmail and make good use of it. I also love the label system. Often, I will label an incoming e-mail "TO DO" and then slide it into a folder I've created. Could you consider a feature that allows the user to "place a sticky note" (so to speak) on an e-mail where they can jot something down that reminds them what they need to do next with that e-mail??

I picture opening my "TO DO" (labeled) e-mails one by one and, as I do, the "sticky note" reminds me what it was that I wanted to do next with that item.

I hope this makes sense and thank you for considering the idea.