docker-mailserver/docs/content/config/security/ssl.md

There are multiple options to enable SSL:

• using letsencrypt (recommended)
• using self-signed certificates with the provided tool

After installation, you can test your setup with checktls.com.

Let's encrypt (recommended)

To enable Let's Encrypt on your mail server, you have to:

• get your certificate using letsencrypt client
• add an environment variable SSL_TYPE with value letsencrypt (see docker-compose.yml.dist)
• mount your whole letsencrypt folder to /etc/letsencrypt
• the certs folder name located in letsencrypt/live/ must be the fqdn of your container responding to the hostname command. The full qualified domain name (fqdn) inside the docker container is build combining the hostname and domainname values of the docker-compose file, e. g.: hostname: mail; domainname: myserver.tld; fqdn: mail.myserver.tld

You don't have anything else to do. Enjoy.

Self-signed certificates (testing only)

You can easily generate a self-signed SSL certificate by using the following command:

docker run -ti --rm -v "$(pwd)"/postfix/ssl:/ssl -h mail.my-domain.com -t tvial/docker-mailserver generate-ssl-certificate

# Press enter
# Enter a password when needed
# Fill information like Country, Organisation name
# Fill "my-domain.com" as FQDN for CA, and "mail.my-domain.com" for the certificate.
# They HAVE to be different, otherwise you'll get a `TXT_DB error number 2`
# Don't fill extras
# Enter same password when needed
# Sign the certificate? [y/n]:y
# 1 out of 1 certificate requests certified, commit? [y/n]y

# will generate:
# postfix/ssl/mail.my-domain.com-key.pem (used in postfix)
# postfix/ssl/mail.my-domain.com-req.pem (only used to generate other files)
# postfix/ssl/mail.my-domain.com-cert.pem (used in postfix)
# postfix/ssl/mail.my-domain.com-combined.pem (used in courier)
# postfix/ssl/demoCA/cacert.pem (certificate authority)

Note that the certificate will be generate for the container fqdn, that is passed as -h argument. Check the following page for more information regarding postfix and SSL/TLS configuration.

To use the certificate:

• add an DMS_SSL=self-signed to your container environment variables
• if a matching certificate (files listed above) is found in postfix/ssl, it will be automatically setup in postfix and courier-imap-ssl. You just have to place them in postfix/ssl folder.

Testing certificate

From your host:

docker exec mail openssl s_client -connect 0.0.0.0:25 -starttls smtp -CApath /etc/ssl/certs/

or

docker exec mail openssl s_client -connect 0.0.0.0:143 -starttls imap -CApath /etc/ssl/certs/

And you should see the certificate chain, the server certificate and:

Verify return code: 0 (ok)