You can quickly reply to an email with quick replies. If you add the 'Quick reply' action to the swipe actions (View), you can swipe, tap on 'Quick reply' and choose the reply to send. This will send a new email with your quick reply.
You can edit or add your own quick replies in the left side panel - Settings - Quick replies. Use the "+" button int he top of the Quick replies screen to add new replies. Tap on the reply text to edit it.
Quick replies can also be used with the Apple Watch app.