Boomerang for Gmail lets you add notes (that only you can see) to emails you snooze or schedule. It's a perfect way to remember important details or context that you can't or don't want to include within an email.
Sometimes its useful to have context about an email or contact that isn't (or can't be) in an email chain you're replying to.
Enter Boomerang for Gmail's Notes feature. Boomerang not only lets you schedule emails and set up follow-up reminders, but it also lets you attach notes to any of these emails as well.