from threading import Thread

def send_async_email(msg):
    sleep(10)
    print('Sending message')
    mail.send(msg)

def send_email(subject, sender, recipients, text_body, html_body):
    msg = Message(subject, sender = sender, recipients = recipients)
    msg.body = text_body
    msg.html = html_body
    thr = Thread(target = send_async_email, args = [msg])
    thr.start()

async def send_async_mail(app, msg):
    with app.app_context():
        mail.send(msg)


def send_mail(subject, sender, recipients, text_body, html_body):
    msg = Message(subject, sender=sender, recipients=recipients)
    msg.body = text_body
    msg.html = html_body
    mail.send(msg)
    send_async_mail(app, msg)

RuntimeWarning: coroutine 'send_async_mail' was never awaited send_async_mail(app, msg)
RuntimeWarning: Enable tracemalloc to get the object allocation traceback