This example uses the python 2.7 environment, and python 3 should operate similarly.
Two packages, smtplib and email, are required.

Send text type mail

Here's an example of sending text messages (using SMTP from NetEase 163):

# -*- coding: UTF-8 -*-

import smtplib
from email.mime.text import MIMEText
from email.header import Header
from email.utils import formataddr

# Third-party SMTP
mail_host = "smtp.163.com" # SMTP server
mail_user = "sender@163.com" # Sender Mailbox
mail_pass = "******" # Client Authorization Number for Mailbox

sender = "sender@163.com" # Sender of mail
recivers = ["bob@qq.com", "someone@gmail.com"] # Mail recipient, you can specify more than one

# Three parameters: the first is the text content, the second formats the text, and the third sets the character encoding
message = MIMEText('Python Mail Sending Test', 'plain', 'utf-8')
message['From'] = sender;
# == message['From'] = formataddr(['sender', sender])
message['To'] = ", ".join(recivers)
# == message['To] = formataddr(['ok', ', '.join(recivers)])
subject = 'Python Mail Test'
message['Subject'] = Header(subject, 'utf-8')

try:
    smtpObj = smtplib.SMTP()
    smtpObj.connect(mali_host， 25) # Connect SMTP, port 25
    smtpObj.set_debuglevel(1)
    smtpObj.login(mail_user, mail_pass)
    smtpObj.sendmail(sender, recivers,message.as_string())
    print "emails send successfully"
except smtplib.SMTPException:
    print "Error:cannot send emails"
smtpObj.quit() # Close Connection

It looks like NetEase has a hole in its SMTP. Messages ['From'] and messages ['To'] need to be consistent with sender and recivers. Titles and content should not have sensitive words as much as possible. Otherwise, they will be judged spam by the server. For the first time, I was blocked because Subject had "SMTP" (funny.gif).
If the message fails to send, you can check the status code returned by the set_debuglevel() function to determine why.

Send mail in HTML format

Unlike sending text, setting _subtype in MIMEText to HTML and sending HTML with pictures also creates an instance of MIMEMultipart().

# -*- coding: UTF-8 -*-

import smtplib
from email.header import Header
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart
from email.mime.image import MIMEImage

# Third-party SMTP
mail_host = "smtp.163.com" # SMTP server
mail_user = "sender@163.com" # Sender Mailbox
mail_pass = "******" # Client Authorization Number for Mailbox

sender = "sender@163.com"
recivers = ["bob@qq.com", "alice@qq.com"]  # Receive mail

msg = MIMEMultipart('related')
msg['From'] = sender
msg['To'] = ", ".join(recivers)
subject = 'HTML 1m4g3'
msg['Subject'] = Header(subject, 'utf-8')
mail_msg = """
<h1>HTML image test</h1>
<p><a href="http://www.baidu.com">learn more</a></p>
<p><img src="cid:image1"></p>
"""
msgAlternative = MIMEMultipart('alternative')
msg.attach(msgAlternative)
msgAlternative.attach(MIMEText(mail_msg, 'html', 'utf-8'))

# Specify the picture of the current directory
fp = open('test1.gif', 'rb')
msgImage = MIMEImage(fp.read())
fp.close()

# Define picture ID s, referenced in HTML
msgImage.add_header('Content-ID', '<image1>')
msg.attach(msgImage)

try:
    smtpObj = smtplib.SMTP()
    smtpObj.connect(mail_host, 25)  # 25 SMTP Port
    smtpObj.set_debuglevel(1)
    smtpObj.login(mail_user, mail_pass)
    smtpObj.sendmail(sender, recivers, msg.as_string())
    print "emails send sucessfully"

except smtplib.SMTPException:
    print "Error:cannot send emails"
smtpObj.quit()    

Send mail with attachments

# -*- coding: UTF-8 -*-

import smtplib
from email.header import Header
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart
from email.mime.image import MIMEImage

# Third-party SMTP
mail_host = "smtp.163.com"  # Set up the server
mail_user = "sender@163.com"
mail_pass = "******"

sender = "sender@163.com"
recivers = ["bob@qq.com", "alice@qq.com"]  # Receive mail

# Create attachment instance
msg = MIMEMultipart()
msg['From'] = sender
msg['To'] = ", ".join(recivers)
subject = 'Mail Attachments'
msg['Subject'] = Header(subject, 'utf-8')
# Mail body:
MIMEText('The message body is as follows', 'html', 'utf-8')

# Construct an attachment to transfer the txt file in the current directory:
att1 = MIMEText(open('test1.txt', 'rb').read(), 'base64', 'utf-8')
att1["Content-Type"] = 'application/octet-stream'
# The filename here can be named any way you like, as it will appear in the mail
att1["Content-Disposition"] = 'attachment;filename="test_1.txt"'
msg.attach(att1)

try:
    smtpObj = smtplib.SMTP()
    smtpObj.connect(mail_host, 25)  # 25 SMTP Port
    smtpObj.set_debuglevel(1)
    smtpObj.login(mail_user, mail_pass)
    smtpObj.sendmail(sender, recivers, msg.as_string())
    print "emails send sucessfully"

except smtplib.SMTPException:
    print "Error:cannot send emails"

smtpObj.quit()

Sending normal returns 250 status codes:

554 returned from send failure:

summary

One way to do this is to find out how the smart contract CTF flag s out of mailbox, the other is to supplement programming (you type code like Cai Xukun.gif).
All in all, there were many pits in it, and most of them were 554 returned (probably intercepted for spam), probably because some of the Subject's words didn't match (like "test" would be intercepted).Also, when sending in bulk, recivers is a list, so the message['To'] needs to be converted to a string with commas and join() functions.
over!