Sending mail using flask + blueprint

You should use app_context() like this:

from flask import current_appfrom flask_mail import Message, Mailwith current_app.app_context():    mail = Mail()    mail.send(msg)

more info https://flask.palletsprojects.com/en/1.1.x/extensiondev/

The following code is render HTML template for sending mail body including styles or formatted HTML.

from <project-module> import mailtoken = user.get_reset_token()msg = Message('Password Reset Request', sender='<sender-mail>', recipients=[user.email])msg.html = render_template('email_templates/password_reset.html',                               home_link=url_for('home.index', _external=True),                               reset_link=url_for(                                   'account.reset_token', token=token, _external=True),                               name=user.name,                               email=user.email)mail.send(msg)

As you mentioned above code. I have been twice initialize Mail object, which is un-necessary.

@main.route('/mail')def sendmail():    receivers = ['<Receiver 1>', '<Receiver 2>']     mail = Mail()    with mail.connect() as conn:        for receiver in receivers:            msg = Message(                        subject='subject test',                        recipients=[receiver],                        html='<h1>Hi~~</h1>')            mail.send(msg)    return 'ok'

Not a direct answer to your question, but actually I don't think you need to initialize the mail twice.

If you would initialize the mail in app/main/mail.py, there's no point having mail.init_app(app) in app/__init__.py because you never imported it.

Otherwise, in app/main/mail.py I would do import app.mail without creating another mail so that you won't have this config issue in the first place.