SMTPSenderRefused error. How to fix?

Question

Python

I am writing a bulk mailing program. Here is the code

#!/usr/bin/python3
# -*- coding: utf-8 -*-
import smtplib
from email.mime.text import MIMEText

print('Войдите в свой аккаунт')
username = input("Username: ")
password = input("Password: ")
mail_sender = username
mail_receiver_input = input("Введите путь к файлу с адресами получателей: ")
header = input("Введите тему письма: ")
message_input = input("Введите путь к файлу с образцом письма: ")
server = smtplib.SMTP('smtp.gmail.com:587')
server.starttls()
server.ehlo()
try:
    server.login(username, password)
except smtplib.SMTPAuthenticationError:
    print("Неверный пароль или логин")

try:
    for mail_receiver in open(mail_receiver_input, 'r'):
        message = open(message_input, 'r').read()
        mag = MIMEText(message, 'html', 'utf-8')
        mag["Subject"] = header
        mag['From'] = mail_sender
        mag['To'] = mail_receiver
        server.sendmail(mail_sender, mail_receiver, mag.as_string())
except FileNotFoundError:
    print("Не найдено файла или папки")
server.quit()

With a small number of emails, everything works fine, but if you feed her 100+ emails, this error pops up

smtplib.SMTPSenderRefused: (421, b'4.7.0 Try again later, closing connection. (MAIL) p89-v6sm20290047wrc.97 - gsmtp', '[email protected]')

Off. The documentation says that the error means "Sender's address is rejected". I dug into the Gmail settings and found this item.
The switch is in the "Do not limit" position. So what's the problem? To whom it is not difficult I ask to answer my question.

Answer 1

Imap has nothing to do with it. Gmail will not let you send a lot of emails and will interfere with this in every possible way. Use a dedicated service.
By the way, if you configure DKIM, SPF and DMARC normally, then you can send using regular sendmail.

Answer 2

Go to the site solkogan.ru there is an article about sending mail through python where the author painted everything.