How to secure a mail server using encryption

Last updated on November 25, 2020 by Sarmed Rahman

SSL (Secure Sockets Layer) and its descendant TLS (Transport Layer Security) are the most widely used protocols for encrypting data that is exchanged between a server and a client. These protocols often use X.509 certificates and asymmetric cryptography.

STARTTTLS is another method of securing plain-text communication. This protocol also encrypts data with SSL or TLS, but with the same port as the plain-text protocols, instead of using separate ports for SSL/TLS-encrypted communications. For example, IMAP over STARTTLS uses the same port as IMAP (143), while IMAPS (IMAP over SSL) uses a separate port 993.

The previous tutorial describes how to set up a mail server running on Postfix and Dovecot, but the security aspect was not covered. In this tutorial, we demonstrate how to add security to a mail server through TLS/SSL-based encryption.

Certificates needed for TLS/SSL can be self-signed, signed by a free certification authority (e.g., CAcert) or signed by a commercial authority (e.g., VeriSign), and can be generated with utilities like OpenSSL. We are going to use a self-signed certificate in this tutorial.

Enable TLS Encryption for Postfix

A self-signed certificate can be created with the following command.

# openssl req -new -x509 -days 365 -nodes -out /etc/ssl/certs/postfixcert.pem -keyout /etc/ssl/private/postfixkey.pem

The above command requests a new certificate which is of type X.509, and remains valid for 365 days. The optional -nodes parameter specifies that the private key should not be encrypted. An output certificate file is saved as postfixcert.pem, and an output key file as postfixkey.pem.

All necessary values for the certificate can be given:

Country Name (2 letter code) [AU]:BD 
State or Province Name (full name) [Some-State]:Dhaka 
Locality Name (eg, city) []:Dhaka 
Organization Name (eg, company) [Internet Widgits Pty Ltd]:
Organizational Unit Name (eg, section) []:Example.tst 
Common Name (e.g. server FQDN or YOUR name) []:mail.example.tst 
Email Address []:[email protected] 

Now that the certificate is ready, necessary parameters are adjusted in postfix configuration file.

[email protected]:~# vim /etc/postfix/main.cf
### STARTTLS is enabled ###
smtpd_tls_security_level = may 

smtpd_tls_received_header = yes 
smtpd_tls_auth_only = yes 

### loglevel 3 should be used while troubleshooting ###
smtpd_tls_loglevel = 1

### path to certificate and key file
smtpd_tls_cert_file = /etc/ssl/certs/postfixcert.pem 
smtpd_tls_key_file = /etc/ssl/private/postfixkey.pem 
smtpd_use_tls=yes 

Restart postfix to enable TLS.

[email protected]:~# service postfix restart

At this point, postfix is ready to encrypt data to and from the server. More details about Postfix TLS support can be found in their official README.

Enable SSL Encryption for Dovecot

Configuring dovecot for encryption is similar to postfix.

First of all, a self-signed certificate is created with openssl:

# openssl req -new -x509 -days 365 -nodes -out /etc/ssl/certs/dovecotcert.pem -keyout /etc/ssl/private/dovecotkey.pem

The above command requests a new X.509 certificate which is valid for 365 days. -nodes is an optional parameter which specifies that the stored private key should not be encrypted. An output certificate file will be dovecotcert.pem, and an output key file will be dovecotkey.pem.

All necessary parameters need to be specified in the certificate:

Country Name (2 letter code) [AU]:BD
State or Province Name (full name) [Some-State]:Dhaka
Locality Name (eg, city) []:Dhaka
Organization Name (eg, company) [Internet Widgits Pty Ltd]:
Organizational Unit Name (eg, section) []:Example.tst
Common Name (e.g. server FQDN or YOUR name) []:mail.example.tst
Email Address []:[email protected]

Next, the path to the certificate is added in dovecot configuration.

[email protected]:~# vim /etc/dovecot/conf.d/10-ssl.conf
nssl_cert = </etc/ssl/certs/dovecotcert.pem
ssl_key = </etc/ssl/private/dovecotkey.pem

Finally, dovecot is restarted to enable SSL with the new certificate.

[email protected]:~# service dovecot restart

Thunderbird Mail Client Configuration

The following is a snapshot on how to configure the account in Mozilla Thunderbird.

Troubleshooting

First of all, make sure that all necessary ports are allowed in the firewall.

Second, try telnet to a mail server. You should be able to get through. Some examples are given below for reference.

Connect to IMAPS

$ telnet mail.example.tst 993
Trying mail.example.tst... 
Connected to mail.example.tst. 
Escape character is '^]'. 
exit 
exit 
Connection closed by foreign host. 

Connect to POP3S

$ telnet mail.example.tst 995
Trying mail.example.tst... 
Connected to mail.example.tst. 
Escape character is '^]'. 
exit 
exit 
Connection closed by foreign host. 

Connect to SMTP

$ telnet mail.example.tst 25
Trying mail.example.tst... 
Connected to mail.example.tst. 
Escape character is '^]'. 
220 mail.example.tst ESMTP Postfix (Ubuntu) 

### Command ###
ehlo mail.example.tst 
250-mail.example.tst 
250-PIPELINING 
250-SIZE 10240000 
250-VRFY 
250-ETRN 
250-STARTTLS 
250-ENHANCEDSTATUSCODES 
250-8BITMIME 
250 DSN

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