I dislike not having an easy mechanism to remind myself which outgoing messages require a response (so I can send reminders). My mental accounting says that I'

I dislike not having an easy mechanism to remind myself which outgoing messages require a response (so I can send reminders). My mental accounting says that I've done my part as soon as I respond to an email, but I would like to have something to ping me (or give a list of pings) when I don't have responses yet. I have to believe this already exists somewhere.