[Sesse__]

Use shift-F to mark the email as “flag”. It's exactly the same functionality as Gmail's starring, and maps to it over IMAP.

[jlokier]

That's not the same function as "remind me later", which actually does remind you at a future time.

I use shift-F in Mutt to flag emails. I then lose track of them, because I have too many flagged emails!

Task-based tagging might be better (using Notmuch) but I haven't found a good workflow for this yet.