Postfix mail server configuration in Linux step by step

Postfix is an open source mail transfer agent (MTA) for delivering emails in Linux. In this article, we will look at how to install and configure Postfix in CentOS Linux.

Postfix mail server configuration in CentOS Linux step by step

Here are the steps to install & configure Postfix mail server.

1. Update system

Open terminal and run the following command to update Ubuntu system.

# dnf update

Also remove any other mail transfer agent (MTA) that may cause conflict with postfix. Here is an example to remove sendmail.

# dnf remove sendmail

2. Set hostname and add host

Run the following command to set hostname to your domain/website.

# hostnamectl set-hostname www.example.com
# exec bash

Add system’s hostname and IP address in /etc/hosts file.

# vim /etc/hosts
192.168.15.25   www.example.com

3. Install Postfix

Install Postfix with the following command.

# dnf install postfix

4. Start & Enable Postfix service

Run the following commands to start postfix and enable it to autorun during system reboot.

# systemctl start postfix
# systemctl enable postfix

5. Install mailx email client

We need to install mailx service to be able to use postfix. Install it with the following command.

# dnf install mailx

6. Configure Postfix mail server

Open Postfix configuration file in a text editor.

$ sudo vi /etc/postfix/main.cf

Replace myhostname, mydomain with your domain, and also change IP address mentioned in mynetworks as per your requirement.

myhostname = www.example.com
mydomain = example.com
myorigin = $mydomain
inet_interfaces = all
inet_protocols = all
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
mynetworks = 192.168.1.0/24, 127.0.0.0/8
home_mailbox = Maildir/

Save and close the file. Restart postfix mail server.

# systemctl restart postfix

7. Test Postfix Mail Server

We need to create Linux user to test our configuration.

# useradd postfixuser
# passwd postfixuser

Next, use the telnet command to send email from your logged in user to the newly created user.

# telnet localhost smtp
or
# telnet localhost 25

If telnet is not installed on your system, install it with the following command.

# dnf install telnet -y

When you issue the telnet command, you will see the following kind of output.

[root@server1 ~]# telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 www.example.com ESMTP Postfix

It indicates that you are able to connect to postfix mail server successfully. Next, run the following command.

# ehlo localhost

You will see the following output.

250-www.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250 SMTPUTF8

Next, run the following commands in bold to compose and send the email in terminal. When you issue quit command, your email will be sent.

mail from:<ubuntu>
250 2.1.0 Ok
rcpt to:<postfixuser>
250 2.1.5 Ok
data
354 End data with <CR><LF>.<CR><LF>
Hello, Welcome to my mailserver (Postfix)
.
250 2.0.0 Ok: queued as B56BF1189BEC
quit
221 2.0.0 Bye
Connection closed by foreign host

If the mail was sent successfully, you can view it in your home directory.

# ls /home/postfixuser/Maildir/new
1573580091.Vfd02I20050b8M635437.www.example.com
#

You can view the email content with cat command.

# cat /home/postfixuser/Maildir/new/1573580091.Vfd02I20050b8M635437.www.example.com

You can view your mail server’s logs at /var/log/maillog

# tail -f /var/log/maillog

If you want to check your server’s mail queue, use the mailq command

# mailq
Mail queue is empty

8. Secure Mail Server

It is advisable to secure your mail server using an SSL certificate. Here is a command to create a private key and certificate signing request.

# openssl req -nodes -newkey rsa:2048 -keyout mail.key -out mail.csr

Run the following command to generate self-signed certificate.

# openssl x509 -req -days 365 -in mail.csr -signkey mail.key -out mail.crt

Copy the SSL certificate to /etc/postfix directory.

# cp mail.key mail.crt /etc/postfix

Run the following command to open postfix configuration file.

# vi /etc/postfix/main.cf

Add the following lines to update SSL certificate location in Postfix configuration file.

………
smtpd_use_tls = yes
smtpd_tls_cert_file = /etc/postfix/mail.crt
smtpd_tls_key_file = /etc/postfix/mail.key
smtpd_tls_security_level = may
………

Save and close the file. Restart postfix server.

# systemctl restart postfix

Here’s a simple command to test if secure emails work on your server.

# echo "test email" | mailx -s "Test email from Postfix MailServer" -r ubuntu@example.com postfixuser@example.com

To send emails to outside users, replace postfixuser@example.com with external email address.

That completes postfix mail server configuration. In this article, we have learnt how to install, setup & configure postfix mail server, and also secure it.