Sometimes when reading emails, you come across messages that require further action at a later time. The default Mail app on your iPhone and iPad makes it easy to flag a message for later review. If you think you may forget to respond to someone or just need an easy way to access that email at any time, flag it.
Launch the Mail app from the Home screen of your iPhone or iPad.
Enter the Inbox that contains the message you'd like to flag.
Find the specific email message in your list of messages and then tap the Edit button in the upper right hand corner.
Tap on the message or messages you'd like to flag so they're highlighted.
Now tap on the Mark button in the lower right hand corner and choose Flag.
You'll now see a flag next to the message inside your inbox and you'll also notice the addition of a flagged section to your inbox list for easy access later.
If you ever want to unflag a message, you'll repeat the same steps as above and choose Unflag the second time around.