With watchOS 2, Apple added some features to the Mail app that make it possible for you to actually respond to email right from Apple Watch. Similar to responding to text messages on Apple Watch, users can now use voice dictation, send an emoji, or respond with a preset default reply.
Additionally, Facebook recently updated its Messenger app to work on Apple Watch. So, now you can read and reply to chats with Facebook contacts.

Apple provides room for about 20 default replies for Messages and about a dozen for Mail. Facebook provides six. You are not confined to the premade responses, however, and you can customize them to say whatever you’d like.
We’ve got a quick tutorial explaining how to customize your quick responses to fit your personality. The steps are very similar for customizing default responses in all three apps.

Messages

On iPhone, open the Apple Watch app and navigate to the My Watch tab.

Select Messages from the menu.

Tap “Default Replies.”

Tap a premade response and replace it with your own.

Mail

On iPhone, open the Apple Watch app and navigate to the My Watch tab.

Select Mail from the menu.

Tap “Default Replies” under the Mail Settings section.

Tap a premade response and replace it with your own.

Facebook Messenger

On iPhone, open the Messenger app and tap the Settings icon in the bottom right corner of the screen.

Select Apple Watch from the menu.

Tap a premade response and replace it with your own.