First we need to create our certificates. To create a certificate authority download the openssl-perl package through Yum:
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yum install openssl-perl
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Then issue the following command to create the CA certificate.
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./CA.pl -newca
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After this process is done we need to create the certificate request and key. Once the certificate request has been generated you then need to have the pem file signed by the CA.
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openssl req -new -days 365 -newkey rsa:2048 -keyout newkey.key -out newreq.pem
openssl ca -out post_signed_cert.pem -infiles newreq.pem
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Create a directory on the Postfix server under the /etc/pki directory called postfix, place these files there, and change the permissions on the files.
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chmod 0400 *
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Add the following lines at the bottom of the main.cf file for Postfix
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#SASL
smtpd_sasl_auth_enable = yes
broken_sasl_auth_clients = yes
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_security_options = noanonymous
smtpd_recipient_restrictions =
permit_mynetworks,
permit_sasl_authenticated,
reject_unauth_destination

#TLS
smtpd_tls_key_file = /etc/pki/postfix/private/postkey.key
smtpd_tls_cert_file = /etc/pki/postfix/certs/post_signed_cert.pem
smtpd_tls_loglevel = 1
smtpd_tls_session_cache_timeout = 3600s
smtpd_tls_session_cache_database = btree:/var/spool/postfix/smtpd_tls_cache
tls_random_source = dev:/dev/urandom
smtpd_tls_auth_only = yes
smtpd_tls_security_level = may

#HELO Restrictions
smtpd_delay_reject = yes
smtpd_helo_required = yes
smtpd_helo_restrictions =
permit_mynetworks,
reject_non_fqdn_helo_hostname,
reject_invalid_helo_hostname,
permit
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Next uncomment the submission line in the master.cf file.
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smtp inet n - n - - smtpd
submission inet n - n - - smtpd
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We now need to tie it into Dovecot. Go to the auth default section of the configuration file and add/edit the following lines.
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auth default {
mechanisms = plain login
passdb pam {
}
userdb passwd {
}
user = root
socket listen {
client {
path = /var/spool/postfix/private/auth
mode = 0660
user = postfix
group = postfix
}
}
}
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Restart the Postfix and Dovecot services.
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service postfix restart
service dovecot restart
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In order to test to make sure SSL/TLS is working we will need to telnet to the port and run a few commands.
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$ telnet localhost 25
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
220 smtp.example.com ESMTP Postfix
ehlo smtp.example.com
250-smtp.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
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What you are looking for is STARTTLS, this will tell you that SSL/TLS is activated. To make sure that the certificates are working correctly type: STARTTLS which should say 220 2.0.0 Ready to start TLS. If it says anything else then there is something wrong and you will need to go back and fix it.