How to create a Email Server under Linux

Email Platform 

Although this email service can’t send email to other people due to server providers email port block, it can be used to receive email.

There are 2 options, but I decided to use other solution since my VPS has limited memory and disk storage:

• https://github.com/mailcow/mailcow-dockerized
• https://github.com/Mailu/Mailu

I’m going to make a SMTP(Simple Mail Transfer Protocol) service by myself:

• https://stackoverflow.com/a/10850619/8667243
• https://aiosmtpd.readthedocs.io/en/latest/migrating.html
• https://docs.python.org/3/library/email.examples.html
• https://petri.com/configure_mx_records_for_incoming_smtp_email_traffic/
• https://stackoverflow.com/questions/7297373/smtp-directly-to-a-hosts-mx-record

How I did 

For the python part, you can simply use python to set up a SMTP service, which will listen to a port, for google, they use 25.

Then if you want the outside world to be able to send email to you, you have to set up a MX record and an A record.

It is normally like this:

MX  your_domain.com          mail.your_domain.com
A   mail.your_domain.com     192.168.1.1(your server IP address)
TXT mail.your_domain.com v=spf1 ip4:192.168.1.1(your server IP address) -all

Then you could send email to your email service by specify the sender email as admin@mail.your_domain.com

And if you want to check if an email was sent from the real user, you can do a check like this:

1. Get source_ip from the email, for example, 192.168.1.1
2. Get the TXT record from a domain after @, for example, mail.your_domain.com. The result will give you an IP address or IP Range.
3. If the ((source_ip == txt record ip) or (source_ip in text record ip range)), then that email was sent from mail.your_domain.com for sure.

email service example

import smtpd
import asyncore
from typing import Any

class CustomSMTPServer(smtpd.SMTPServer):
    def process_message(self, peer: Any, mailfrom: Any, rcpttos: Any, data: Any, mail_options: Any = None,rcpt_options: Any = None):
        print ('Receiving message from:', peer)
        print ('Message addressed from:', mailfrom)
        print ('Message addressed to  :', rcpttos)
        print ('Message length        :', len(data))
        print(data)
        return None

server = CustomSMTPServer(('0.0.0.0', 25), None) #type: ignore

asyncore.loop()

email sender example

import smtplib
from email.message import EmailMessage

msg = EmailMessage()
msg['Subject'] = f'Hi, you!'
msg['From'] = "yingshaoxo@gmail.com"
msg['To'] = "lili@gmail.com"

s = smtplib.SMTP('0.0.0.0', port=25)
s.send_message(msg)
s.quit()

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