Add Flask-Mail to app instance.

app/__init__.py

 from config import config
 from flask import Flask
 from flask_login import LoginManager
+from flask_mail import Mail
 from flask_sqlalchemy import SQLAlchemy
 
 
 db = SQLAlchemy()
+mail = Mail()
 
 login_manager = LoginManager()
 login_manager.login_view = 'auth.login'
@@ -17,6 +19,7 @@ def create_app(config_name):
 
     db.init_app(app)
     login_manager.init_app(app)
+    mail.init_app(app)
 
     from .auth import auth as auth_blueprint
     app.register_blueprint(auth_blueprint, url_prefix='/auth')