Using SMTP and the Gmail API (Could you please demonstrate both methods?), I need to send thousands of emails, each customized according to a name and containing an attachment. How can I accomplish this process as quickly and efficiently as possible?

My current method, in its simplest form, is as follows:

from os import path
import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.application import MIMEApplication
from traceback import print_exc
from copy import deepcopy
from importlib import util
from time import perf_counter

class EmailSender:
    def __init__(self, username, client, data, message, subject, html, attachments):
        # Constructor details...

    def send_email(self, index, receivers, message):
        try:
            # Sending email details...

        except Exception as e:
            print_exc()

    def prepare_and_send_emails(self):
        for index, i in self.data.items():
            tmp_msg = deepcopy(self.message)
            self.send_email(index, i, tmp_msg)

    @staticmethod
    def multi_replace(string, replacements):
        # Multi-replace function...

def call_func(script_path, func=None, arguments=None):
    # Function to call external scripts...

time = perf_counter()
client = smtplib.SMTP('smtp.gmail.com', 587)
client.starttls()
client.login('[email protected]', 'rvfc twfa xzve yxah')
data = {0: ['[email protected]', [100], {'Name': 'Johan'}], 1: ['[email protected]', [100], {'Name': 'Max'}],
        # Data dictionary...

message = MIMEMultipart()
subject = "Mail to ${Name}"
html = "Hello Dear ${Name}"
attachments = { # Attachment details... }

non_script_attachments = [key for key, value in attachments.items() if not value['script_name']]
script_attachments = {key: value for key, value in attachments.items() if value["script_name"]}

# Attachment handling...

sender = EmailSender('[email protected]', client, data, message, subject, html, script_attachments)
sender.prepare_and_send_emails()
print(perf_counter()-time)

But this is so slow. When I use threads like this:

    def prepare_and_send_emails(self):
        threads = []
        for index, i in self.data.items():
            tmp_msg = deepcopy(self.message)
            thread = threading.Thread(target=self.send_email, args=(index, i, tmp_msg))
            threads.append(thread)
            thread.start()

        for thread in threads:
            thread.join()

I encounter various errors. I used similar code within Google, and it gave errors when using threads there too.

When I try open new SMTP server for each thread this is even more slower than normal code.

How can I use something like threads for sending emails?