Starting from iOS 8, you can actually set “reply alerts” in the Mail app. The reply alerts is a feature that notifies you when you receive a reply for a particular email.
This means you don't have to keep checking your Mail every once in a while in anticipation of the reply. When you get the reply, you'll get a notification almost instantly.
Here's How to Set up Mail Reply Notifications on iPhone/iPad in iOS 8
Setting it up is Quick and Easy:
Step #1. Open the Mail app on your iPhone/iPad
Step #2. Swipe to left on the mail conversation/thread to reveal the options
Step #3. Tap on More
Step #4. From the options, tap Notify Me…
Once you set a “Notify Me” alert, the email thread will now feature a bell icon which tells you that the thread/conversation will send notifications to you when you get a reply.
You can do this with individual emails as well as conversation threads. Depending on where you swipe, the notification alert gets applied to a particular email or a thread.
The Reply Notification feature is great for a variety of reasons but one of the things that I'd like to add is that it comes close on the heels of other mail apps gaining traction and becoming popular. Especially since we're talking about no push (only fetch) notifications these days. When you have this feature on your Mail app, it really changes the way you approach mail. You are no longer spending your time checking for replies. You're just waiting for the inevitable ping.
To Disable Notifications:
Step #1. Swipe on the mail thread/conversation
Step #2. Tap on More
Step #3. Tap on Stop Notifying Me
Step #1. Open the Mail
Step #2. Tap on the Flag icon
Step #3. Now tap on Notify Me.
To stop notifications, repeat the same and tap on Stop Notifying me.