If you find yourself regularly participating in ongoing email conversations, you may find it handy to get more proactive notifications when somebody sends a reply in an existing thread. iOS 6 introduced a “VIP List” feature that lets you get notifications when messages come in from specific people, but that’s not always useful if you only want to enable notifications on an ad-hoc basis, so the Mail app in iOS 8 takes this a step further with the ability to enable notifications for a specific thread.
Typically, this is done when sending a message, and is simply a matter of tapping the small bell icon that appears to the right of the subject line once you’ve placed the cursor in there. You’ll see a pop-up confirming that this is what you want to do, and then once you’ve sent the message, any replies to that message will automatically trigger the “Thread Notification” — a separate set of notifications, much like the VIP list, that can be configured in your Notification Center settings — this means that thread replies can trigger a different alert sound and bring up lock screen and banner notifications.
If you’ve forgotten to enable a Thread Notification for a message you’ve already sent, or want to add one to a message you’ve received (in case somebody else replies to a group discussion, for instance), you can do this by tapping on the flag button in the lower-left corner of the message window, and then choose the “Notify Me…” option.
Messages that have Thread Notifications enabled will show the small bell icon by the sender’s name in the message list, by the subject line in the message, and in the gray heading bar of a threaded conversation view. Should you decide you no longer want to receive these notifications, you can toggle them off in much the same way as turning them on: tap the flag button when viewing an individual message in the thread, and tap “Stop Notifying.”