Simple GMail Notes is the solution I've been looking for forever. Most of my life is somehow in GMail (sadly). For years, when I've wanted to make sure I recalled why I had or had not taken an action in response to an email, I forwarded the email to myself with my notes. Simple Notes is a far better tool for this purpose. I can drop a note on an email of anything I want to remember about that email and not only is it there when I go back to it, but there is also what is essentially a yellow flag next to the email (actually part of the note) that differentiates these emails from others.

As probably implied, I am not an empty-mailbox kind of user. I also have a busy life and a forgetful nature. I probably retain too much, but I have thousands of emails in my inbox and use search to find and recall communications I want or need to revisit. Having the note remind me of something not explicitly in the message is a life-saver. (Maybe that's a little dramatic, but it feels that way at times.)

My Palm handheld in the early 2000s had this functionality and I've been looking for it every since. The add-on works flawlessly and I like having it saved on my computer rather than in a third-party's server.

Now I need the same thing for my telephone calls.

A must have every time I start on a new computer. Attaching notes to email messages simplifies tracking important messages.

I've been waiting for this for many years!

Terrific! I waited this possibility for years!

This is a must have gmail addon. We couldn't do without it.

I am so very happy I have discovered this. I have no idea whether it is going to be buggy, etc - what I do know is that it finally allows me to master my inbox, showing me notes on top of subject lines WITHOUT MY NEEDING TO OPEN MAIL! So easy and convenient.

This is by far the best extension I have used for this. I use it constantly and will even pay for it if I needed to. Under active development which means that any bugs get fixed quickly.

Great app, excellent support. Keeping up with browser and gmail changes is not easy. Invaluable app for the way I use gmail.
Now end-March 2020 - still invaluable to me.
Firefox required the developers to jump through hoops recently on security and other issues. No problem.