One of the features introduced with Apple's WatchOS 2 update was the ability for users to reply to email messages directly from the Apple Watch. This tutorial will explain how to use the Mail app on the Apple Watch to send quick replies to your emails.
Respond to Emails on the Apple Watch
From the homescreen of your Apple Watch, open the Mail app and select the desired message. Hold your finger down on the message for a few seconds to display the Options menu. Tab on Reply to proceed.
The small screen of the Apple Watch is not very suitable for typing, so you may use one of two methods to send your reply. Tap on the Microphone icon to dictate your reply into the watch, or select the Smiley icon to send an emoji response.
Create Custom Replies Using the Mail App
Your Mail application can store up to 20 custom reply messages (default replies may be edited). Open the Apple Watch app installed on its companion iPhone. Next, open the Apple Watch app, and go to My Watch > Mail > Default Replies. You may edit your custom replies here.