You can always clear the Flag for Recipients check box to customize flags just for you.
This feature requires that you save your sent e-mail messages.
If a reminder is included, the reminder alert activates at the specified time unless the item is opened and the reminder is turned off.
Note: If you want to further customize your flag by specifying Start and Due dates or changing when a reminder alert will appear, follow the steps in the Flag messages for both yourself and others section.
The main difference is that the e-mail is delivered almost instantly, although there can be delays caused by server problems, the fact that a recipient's mail box might be full, and other technical problems that will occur from time to time.