Here's (approximately) what I do:

I move some emails into an "action" email folder. For most of these,
the email itself is the only reminder. For some of them,
I also put a "next action" in another system, referring to the email:
e.g. if it's more important, or urgent, or will take more time to work on,
or has a next-action that isn't immediately obvious on looking at the email,
or just because I feel like writing a reminder to get it off my mind.

I have a daily reminder in my system to review emails in the "action" folder.