https://captio.co/ (https://twitter.com/EmailYourself) to send quick reminders to myself. It hasn't been updated in a while, e.g. no dark mode, so there might be chance for somebody else to improve. I'd value it at $some USD/month but I might in the minority here. It was a single purchase when I installed it.
oh! I do that all the time using gmail haha, thanks!