One reminder is not enough
Snooze, Remind Me, Bubble Up, follow-up reminders: every mail app has one, and they all fire once. Gadfly gives each to-do a deadline and a band that climbs from calm to hot to burning to critical. The closer it gets, the louder it is, and it stays in your Chase list until you finish it or decide to drop it.
Where you will actually see it
A push an hour before it is due, or at your morning briefing. Your most urgent to-dos on the iPhone lock screen and home screen. Read the mail anywhere and the notification clears everywhere, so reminders stay useful instead of turning into noise.
Quiet when you need it
Quiet hours, quiet days and a daily cap keep reminders from becoming spam you learn to ignore. Only critical, overdue to-dos break through.