Create a task in you to-do list each time you "flag" an e-mail and never forget to answer if anymore.
Go to IFTTT website and create an account
Click on your picture profile in the right corner and select "Create" in the list
For IF : search for "email"
Select "Send IFTTT any email"
For THEN : search for "ios reminders"
Select "Add reminder to list"
Complete the form "action fields" as you wish
Click on finish
I therefore decided to search for : ifttt recipes for productivity and chose something simple and useful.
Now you can peacefully put off until tomorrow things you can do today...