Install Your Own Mail Server

The user usually has a mail client or MUA — Mail User Agent (Thunderbird, Mutt, etc…), that can use the POP3 or IMAP protocol to fetch emails from the server.

That mail client connects to the POP3 (TCP 110) or IMAP (TCP 143) port on the server and may or may not send the STARTTLS command that initiates an encrypted connection. It then sends the user's username (which is equal to the email address in our case) and their password. POP3 is less used nowadays and lacks of support for multiple folders on the server.

The client may as well use the secure TLS-encrypted ports directly — 995 for POPs or 993 for IMAPs.

Dovecot sends a query to the MySQL database and verifies that the username and password belong to a known user. If the password is wrong then Dovecot will refuse the login.

As defined in the Devecot configuration file, each user has a directory where emails are stored. So if the user is named vselvarajah@atomicl.net, Dovecot will look for the mail directory in /var/vmail/atomicl.net/vselvarajah/Maildir/… and save the email in the corresponding mail directory.

NOTE: We can also integrate a webmail service so users can send/read mails using the browser but we are not going to do that here.

The user writes the email using a mail user agent (MUA) such as Thunderbird, Mutt, etc. And clicks on "Send". The mail client establishes an SMTP connection to your Postfix server on port 25 or 587. In most cases, the user's internet service provider blocks port 25 to prevent spam, so users use port 587.

To ensure that the user is allowed to send emails to other SMTP servers through your system, a username (or email address) and password are required.

In the guide we instruct Postfix to use encryption for authentication for security reasons, but this is completely optional.

Your Postfix server checks the email address and password in the database. Or it can delegate to another service like Dovecot to handle authentication.

Then Dovecot sends a query to the MariaDB database to check if the email address and password are correct and tells Postfix the result.

Postftix knows now that it is authorized to send the email on behalf of the user. It tells the user that it successfully accepted the email. The email is put into Postfix’s mail queue for further processing. Postfix will now query a DNS server to determine mail server of recipient. As the recipient has an ...@recipient-domain.com email address it checks the MX record of the recipient-domain.com domain and then gets the respective IP address.

Postfix now knows which mail server to send the email to. It opens an SMTP connection and delivers the email.

1. SENDER's SMTP: Query DNS server for email server of the domain atomicl.net.
2. DNS: Checks MX record of atomicl.net domain for mails servers and sends the IP address.

1. SENDER's SMTP: Connects to that IP on TCP port 25 which is by default used by SMTP server (Simple Mail Transport Protocol)
2. MY POSTFIX: Welcome, I am Postfix server. Who is there ? (220 mail.atomicl.net ESMTP Postfix)
3. SENDER's SMTP: Hi, I am a remote server. (EHLO mail.sender-domain.com)
4. MY POSTFIX: Nice to meet you. I can offer you a few features like pipelining and encryption… (STARTTLS, PIPELINING, SIZE 4000000, …)
5. SENDER's SMTP: OK, then we switch to an encrypted connection. (STARTTLS) The connection is now using TLS encryption.
6. SENDER's SMTP: I have an email from user@sender-domain.com (MAIL FROM:<user@sender-domain.com>)
7. MY POSTFIX: I see. (Ok)
8. SENDER's SMTP: This email is for vselvarajah@atomicl.net (RCPT TO:<vselvarajah@atomicl.com>)

1. MY POSTFIX: Hi database. (Connects to TCP port 3306 on the local host to talk to MariaDB.) Could you check if atomicl.net is one of our mail domains ? (SELECT … from virtual_domains …)
2. MY MariaDB: Yes, I have a that domain in my database.
3. MY POSTFIX: Nice. And do you have a mailbox or a forwarding rule for vselvarajah@atomicl.net ? (SELECT … from virtual_aliases/virtual_users …)
4. MY MARIADB: Yes, there is a mailbox for that address.
5. MY POSTIFX: Remote server, the recipient looks good. (Ok)
6. SENDER'S SMTP: Good. Then here’s the actual email. (DATA) The remote server sends the email header and body.

1. MY POSTFIX: Connects to port 11332 on the local host to reach the rspamd.
2. My Postfix: Hi, rspamd. I have a new email. Could you spam check ?
3. MY RSPAMD: Sure. Well, there are a few minor issues. But generally the mail looks good. I suggest you accept it.

1. MY POSTFIX: Hey, remote server. Your email is fine.

1. MY POSTFIX: Uses a socket file at /var/spool/postfix/private/dovecot-lmtp to talk to Dovecot.
2. MY POSTFIX: Hi, Dovecot. Here is a new email for vselvarajah@atomicl.net
3. MY DOVECOT: Got it.

1. MY DOVECOT: Checks for additional Sieve rules and then stores the email on disk at /var/vmail/atomicl.net/vselvarajah/Maildir/INBOX