Reply from Apple Watch. Whether you’re reading the message in the notification or in the Mail app, scroll to the bottom and tap Reply.
You can reply by dictating text or an audio message, sending a static or animated emoji, or choosing from a list of prepared replies. To add your own replies to the list, open the Apple Watch app on iPhone, tap My Watch, go to Messages > Default Replies, then tap a reply to change it.
Reply from iPhone. If you want to type a reply, just wake iPhone and swipe up on the Mail icon in the lower-left corner of the Lock screen.