The trick: when you’re composing either a new email or a reply to an existing message, just tap the subject line.
When you do, a little alert bell will appear to the right of the subject. Go ahead and tap it, then tap the “Notify Me” confirmation button.
Want an alert for an email thread that you haven’t replied to yet? Just swipe it from right to left, tap the More button, then tap the “Notify Me” button.
Now, whenever someone replies to that particular thread, you’ll get an iOS notification on your iPhone or iPad. To manage the type of notification you get (a banner or an alert, with or without sound), tap Settings, Notifications, Mail, then Thread Notifications.
Want to turn the thread alert off? Go to your inbox, swipe the message thread from right to left, tap the More button, then tap “Stop Notifying.”