If you get an email that you want to easily find again later, maybe to follow up, maybe just to keep handy, you can flag it right in your iPhone, iPod touch, and iPad Mail app. Flagged messages are easy to spot, thanks to an icon, and even easier to find, thanks to a dedicated smart mailbox that keeps them all handy.
Once you have a message (or messages) flagged, a special Flagged mailbox will appear in your Mail account list. Tap on it to see a list of all your flagged messages.
If you ever want to unflag a message, you'll repeat the same steps as above and choose Unflag the second time around.