I wonder why a feature available on gmail is not popular and broadly spread. Gmail calls it "snooze":
It is the feature that allows marking an email (either received or to-be-sent... and, why not, any other email!), to set a reminder that will automatically display the marked email at a moment I've set (say, 2 days after, or 2 hours, whatever : whenever I feel I need to follow up if I have got no answer from the correspondant, etc).
My current way to handle these numerous emails I need to follow up is... Post-Its all around my monitor 😞
Of coures, one could dream about a very sophisticaed system, like the one developed for the filters, but I would be VERY happy if I were able to merely set the time where the "sonnzed" email would pop up automatically (or a list...) : the moment I determined as adequate, at the time I wrot 'or received) it and set the snooze.
I guess it could be an add-on, if it proves too complicated to be a feature integrated into Thunderbird ?
Sorry for the Post-It business 😉