If you want to send or receive an email, you should have a mail server. In this post, we will discuss the Linux mail server and how the SMTP (Simple Mail Transfer Protocol) works as well as other mail-related protocols, like Post Office Protocol (POP) and Internet Message Access Protocol (IMAP) and the relationship between them.

Linux SMTP server

SMTP defines how to send mail from one host to another; it is also system independent, which means the sender and receiver can have different operating systems.

SMTP requires only that a server can send straight ASCII text to another server, you can do this by connecting to the server on port 25, which is the standard SMTP port.

Most Linux distros today come with two of the most common implementations of SMTP, which are sendmail and Postfix.

Sendmail is a famous and free mail server, but it has a little complex design and less secure.

The Postfix took mail server implementation one step further; they developed it with security in mind.

Mail service components

The mail service on any mail server has three components:

Mail user agent (MUA): this component that the user sees and interacts with like Thunderbird and Microsoft Outlook, these user agents are responsible for reading mail and allowing you to compose mail.

Mail transport agent (MTA): this component is responsible for getting the mail from one site to another like Sendmail and Postfix.

Mail delivery agent (MDA): this component is responsible for distributing received messages on the local machine to the appropriate user mailbox like postfix-maildrop and Procmail.

Setup Email server

We chose the Postfix mail server, which is very popular and common among system administrators today.

Postfix is the default mail server on most modern Linux distros.

First, check if it is installed on your system or not:

$ rpm -qa | grep postfix

If not installed, you can install Postfix mail server on Red Hat based distros like this:

$ dnf -y install postfix

Then start the postfix service and enable it on system startup:

$ systemctl start postfix $ systemctl enable postfix

On Debian based distros like Ubuntu, you can install it like this:

$ apt-get -y install postfix

It will ask you to select the type of configuration of the Postfix mail server during the installation process.

Among the four choices No configuration, Internet site, Internet with smarthost, Satellite system, and Local only, we will choose No configuration option.

Configure Linux mail server

After installing the Postfix mail server, you will need to configure it; you can find most of its configuration files under the /etc/postfix/ directory.

You can find the main configuration for Postfix mail server in /etc/postfix/main.cf file.

This file contains a lot of options like:

myhostname

You can use this option for specifying the hostname of the mail server. This is the Internet hostname, which Postfix will receive emails on it.

The hostnames could be like mail.example.com, smtp.example.com.

myhostname = mail.example.com

mydomain

This option is the mail domain that you will be servicing, like example.com

The syntax is like this:

mydomain = example.com

myorigin

All emails sent from this mail server will look as though it came from this option. You can set this to $mydomain value.

myorigin = $mydomain

You can use any option value, just precede it with a $ like $mydomain.

mydestination

This option lists the domains that the Postfix server uses for incoming emails.

It can take values like this:

mydestination = $myhostname, localhost.$mydomain, $mydomain, mail.$mydomain, www.$mydomain

mail_spool_directory

There are two modes of delivery that Postfix mail server can use:

Directly to a user’s mailbox.

To a central spool directory, this way, the mail will be in /var/spool/mail with a file for each user.

mail_spool_directory = /var/spool/mail

mynetworks

This option allows you to configure what servers can relay through your Postfix server.

This option should take local addresses like local mail scripts on your server only.

Otherwise, spammers can utilize your mail server to relay their messages and your mail server blacklisted and as a result, you will not be able to receive many emails.

This option has the following syntax:

mynetworks = 127.0.0.0/8, 192.168.1.0/24

smtpd_banner

This variable sets the message the server will send when the client after a successful connection.

It is better to change the banner to something that gives no clue the server you are using.

inet_protocols

This option specifies the IP protocol version used for server connections.

inet_protocols = ipv4

If you change the configuration files for the Postfix mail server, you need to reload the service:

$ systemctl reload postfix

When you type any configuration, you may make a mistake, you can check for errors using the following command:

$ postfix check

This tool will help you find exactly the line and the error so you can fix it.

Checking the mail queue

Sometimes the mail queues are filled up with messages. This happens due to many reasons, like network failure or any reason that can delay mail delivery.

To check the mail queue on your Linux mail server, use the following command:

$ mailq

This command shows the Postfix mail queue.

If your queue is filled up and the message takes several hours to process, then you should flush the mail queue.

$ postfix flush

Now, if you check your mail queue, you should find it empty.

Test Linux mail server

After configuring the Postfix mail server correctly, you should test your mail server.

The first step is to use a local mail user agent like mailx or mail, which is a symlink to mailx.

Try to send a mail to someone else on the same server, if this works, then send to a remote site.

Then try to receive a mail from a remote site.

If you have any problems, check the logs. The log file on Red Hat based distros in /var/log/maillog file and on Debian based distros in /var/log/mail.log file or as defined in the rsyslogd configuration.

I recommend you to review the Linux Syslog Server for a detailed explanation about logs and how to configure the rsyslogd.

If you still have problems, try checking your DNS settings and check your MX records using Linux network commands.

Secure mailboxes from spam using SpamAssassin

One of the ways to fight spam is to scan the mailboxes by some tool, searching for certain patterns associated with spam.

One of the best solutions is SpamAssassin, which is open-source.

You can install it like this:

$ dnf -y install spamassassin

Then start the service and enable it at startup:

$ systemctl start spamassassin $ systemctl enable spamassassin

Once you’ve installed it, you can check the configuration in

/etc/mail/spamassassin/local.cf

file.

SpamAssassin determines if an email is spam or not based on the result of the different scripts scores.

If the message has a higher score, that means a higher possibility of the mail being spam.

In the configuration file, the parameter required_hits 5 indicates that SpamAssassin will mark an email as spam if its score is five or higher.

The report_safe option takes the values 0, 1, or 2. If set to 0, it means email marked as spam is sent as it is, only modifying the headers to show that it is spam.

If it takes the value 1 or 2, SpamAssassin generates a new report message, and it sends the message to the recipient.

The value 1 means the spam message is coded as content message/rfc822, while if the value is 2, that means the message is coded as text/plain content.

The text/plain is safer since some mail clients execute message/rfc822 and could infect the client computer.

Now we need to integrate it into Postfix. The simplest way to do this is probably by using procmail.

We’ll have to create a file, named

/etc/procmailrc

, and add the following content:

:0 hbfw | /usr/bin/spamc

Then we edit Postfix configuration file /etc/postfix/main.cf and change

mailbox_command

like this:

mailbox_command = /usr/bin/procmail

Finally, restart Postfix and SpamAssassin services:

$ systemctl restart postfix $ systemctl restart spamassassin

However, SpamAssassin sometimes does not recognize spam messages that led to mailboxes filled with spam messages.

Fortunately, you can filter messages before they enter the Postfix server using Realtime Blackhole Lists (RBLs). That will decrease the load on your mail server and keep your mail server clean.

Open the configuration file of Postfix server /etc/postfix/main.cf and change smtpd_recipient_restrictions option and add the following options like this:

strict_rfc821_envelopes = yes relay_domains_reject_code = 554 unknown_address_reject_code = 554 unknown_client_reject_code = 554 unknown_hostname_reject_code = 554 unknown_local_recipient_reject_code = 554 unknown_relay_recipient_reject_code = 554 unverified_recipient_reject_code = 554 smtpd_recipient_restrictions = reject_invalid_hostname, reject_unknown_recipient_domain, reject_unauth_pipelining, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, reject_rbl_client dsn.rfc-ignorant.org, reject_rbl_client dul.dnsbl.sorbs.net, reject_rbl_client list.dsbl.org, reject_rbl_client sbl-xbl.spamhaus.org, reject_rbl_client bl.spamcop.net, reject_rbl_client dnsbl.sorbs.net, permit

Then restart your postfix server:

$ systemctl restart postfix

The above RBLs are the common ones; you can find more lists on the web and try them.

Securing SMTP connection

It is better to transfer your SMTP traffic over TLS to protect it from Man In The Middle (MITM) attack.

First, we need to generate the certificate and the key using openssl command:

$ openssl genrsa -des3 -out mail.key $ openssl req -new -key mail.key -out mail.csr $ cp mail.key mail.key.original $ openssl rsa -in mail.key.original -out mail_secure.key $ openssl x509 -req -days 365 -in mail_secure.csr -signkey mail_secure.key -out mail_secure.crt $ cp mail_secure.crt /etc/postfix/ $ cp mail_secure.key /etc/postfix/

Then add the following option to Postfix configuration file /etc/postfix/main.cf:

smtpd_use_tls = yes smtpd_tls_cert_file = /etc/postfix/mail_secure.crt smtpd_tls_key_file = /etc/postfix/mail_secure.key smtp_tls_security_level = may

Finally, restart your postfix service:

$ systemctl restart postfix

Now, you have to choose the TLS on your client when connecting to the server.

You will receive a warning when you send a mail the first time after changing the setting because the certificate is not signed.

Using Let’s Encrypt certificates

Let’s Encrypt is a free SSL certificate provider that enables you to encrypt your traffic.

Instead of using self-signed certificates that annoy your users about trusting them, you can use this good solution.

First, install letsencrypt:

$ yum install letsencrypt

Or if you are using Debian based distro, you can use the following command:

$ apt-get install letsencrypt

Then run letsencrypt like this:

$ letsencrypt certonly --standalone -d yourdomain.com

You should replace yourdomain.com with your actual domain.

After answering the prompted questions about the contact email, the email server domain, and the license, everything should be OK now.

The certificates will be in:

/etc/letsencrypt/live/yourdomain.com/

One last thing you have to do, which is making postfix use those certificates, you can use the following commands:

sudo postconf -e 'smtpd_tls_cert_file = /etc/letsencrypt/live/yourdomain.com/fullchain.pem' sudo postconf -e 'smtpd_tls_key_file = /etc/letsencrypt/live/yourdomain.com/privkey.pem'

Don’t forget to replace yourdomain.com with your actual domain.

Finally, restart your postfix server:

$ systemctl restart postfix

POP3 and IMAP protocol basics

So far we’ve seen how SMTP mail server sends and receives emails without problems, but consider the following situations:

Users need local copies of email for offline viewing.

mbox file format is not supported. The mbox format is used by many mail user agents like mailx and mutt.

Users cannot stay connected to a fast network to grab a local copy to read offline.

Some mail servers don’t give access to the shared mail spool directories for security reasons.

To handle these cases, you should use the mail access protocols.

The most common two popular mail access protocols are Post Office Protocol (POP) and Internet Message Access Protocol (IMAP).

The idea behind POP is very simple: A central Linux mail server remains online all the time and receives and store emails for all users. All received emails are queued on the server until a user grabs them.

When a user wants to send an email, the email client relays it through the central Linux mail server via SMTP normally.

Note that the SMTP server and the POP server can be on the same system without any problem. Most servers do this today.

Features like keeping a master copy of a user’s email on the server were missing, which led to the development of IMAP.

By using IMAP, your Linux mail server will support three modes of access:

The online mode is similar to having direct file system access to the Linux mail server.

is similar to having direct file system access to the Linux mail server. The offline mode is similar to how POP works, where the client is disconnected from the network except when grabbing an email. In this mode, the server normally does not retain a copy of the email.

is similar to how POP works, where the client is disconnected from the network except when grabbing an email. In this mode, the server normally does not retain a copy of the email. The disconnected mode works by allowing users to keep cached copies of their emails, and the server retains a copy of the email.

There are several implementations for IMAP and POP; the most popular one is the Dovecot server, which provides both protocols.

The POP3, POP3S, IMAP, and IMAPS listen on ports 110, 995, 143, and 993 respectively.

Installing Dovecot

Most Linux distros come with Dovecot preinstalled. However, you can install Dovecot in Red Hat based distros like this:

$ dnf -y install dovecot

Debian based distros provide the IMAP and POP3 functionality in two separate packages, you can install them like this:

$ apt-get -y install dovecot-imapd dovecot-pop3d

It will ask you to create self-signed certificates for using IMAP and POP3 over SSL/TLS. Select yes and enter the hostname for your system when prompted.

Then you can run the service and enable it at startup like this:

$ systemctl start dovecot $ systemctl enable dovecot

Configure Dovecot

The main configuration file for Dovecot is

/etc/dovecot/dovecot.conf

file.

Some Linux distros put the configuration under

/etc/dovecot/conf.d/

directory and use the include directive to include the settings in the files.

You can use the following list of the parameters to configure Dovecot:

protocols: the protocols you want to support.

protocols = imap pop3 lmtp

lmtp means local mail transfer protocol.

listen: IP addresses to listen on.

listen = *, ::

The asterisk means all ipv4 interfaces and :: means all ipv6 interfaces

userdb: user database for authenticating users.

userdb { driver = pam }

passdb: password database for authenticating users.

passdb { driver = passwd }

mail_location: this entry in /etc/dovecot/conf.d/10-mail.conf file:

mail_location = mbox:~/mail:INBOX=/var/mail/%u

Secure Dovecot

Dovecot comes with generic SSL certificates and key files. Look at this file:

/etc/dovecot/conf.d/10-ssl.conf

ssl_cert = </etc/pki/dovecot/certs/dovecot.pem ssl_key = </etc/pki/dovecot/private/dovecot.pem

When a user tries to connect to dovecot server, it will show a warning because the certificates are not signed, you can purchase a certificate from a certificate authority if you want.

Or if you go with Let’s Encrypt certificates, you can point to them instead:

ssl_cert = </etc/letsencrypt/live/yourdomain.com/fullchain.pem ssl_key = </etc/letsencrypt/live/yourdomain.com/privkey.pem

Don’t forget to open dovecot server ports in your iptables firewall by adding iptables rules for ports 110, 995, 143, 993, 25.

Then save the rules.

Or if you are using firewalld, you can do the following:

$ firewall-cmd --permanent --add-port=110/tcp --add-port=995/tcp $ firewall-cmd --permanent --add-port=143/tcp --add-port=993/tcp $ firewall-cmd --reload

And again, for troubleshooting, you check the log files /var/log/messages, /var/log/maillog, and /var/log/mail.log files.

Linux mail server is one of the easiest servers to work with, especially the Postfix mail server.

I hope you find the post useful and interesting. Keep coming back.

Thank you.