Add a custom message flag to email
You may know that you can add a flag to your outgoing email messages by clicking the Message Flag icon on the toolbar. Doing this will add a small flag that the recipient can see when they read the email and can also fire a reminder at a nominated time.
But don't be restricted just to the list of suggested flag texts. You can add your own custom text flag simply by typing your chosen words into the Flag to: box. Adding a Due by date/time will ensure a reminder appears when you want it.