When I was looking at mailheaders again (it became kind of a hobby, and this proves you learn from it^^) I was noticing one of my incoming mails was transfered via ESMTPS. So far I knew SMTP and ESMTP but ESMTPS was appearently a new. Turned out it was ESMTP via secure transportlayer, or like RFC 3848 defines it: “The new keyword ‘ESMTPS’ indicates the use of ESMTP when STARTTLS is also successfully negotiated to provide a strong transport”. So I became curious, how can I do that too? After a bit searching I came across the setting smtp_tls_security_level in postfix and yes, after setting it to ‘may’ it did the trick. So now if the server supports STARTTLS he opens a encrypted connection with the remote server for the transfer. You need to set a bit more to make it working without any errors, here is what you need to do on a Ubuntu 10.04 (Debian and others should work similar):
sudo postconf -e 'smtp_tls_security_level = may' sudo postconf -e 'smtp_tls_loglevel = 1' sudo postconf -e 'smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt' sudo service postfix restart
We set only smtp_tls_security_level to ‘may’ cause otherwise with ‘encrypt’ the remote server is forced to support STARTTLS, if he does not the transfer fails. So with may encryption gets used when supported. Loglevel 1 gives you a short notice when a safe connection was established and what cipher got used. Like this:
Mar 14 00:22:08 utgard postfix/smtp: setting up TLS connection to gmail-smtp-in.l.google.com[18.104.22.168]:25 Mar 14 00:22:08 utgard postfix/smtp: Trusted TLS connection established to gmail-smtp-in.l.google.com[22.214.171.124]:25: TLSv1 with cipher RC4-SHA (128/128 bits) Mar 14 00:22:09 utgard postfix/smtp: 6A591E6C2C3: to=, relay=gmail-smtp-in.l.google.com[126.96.36.199]:25, delay=0.9, delays=0.01/0.03/0.13/0.73, dsn=2.0.0, status=sent (250 2.0.0 OK 1331680929 s26si2913819weq.13)
And last but not least, we need to set the path to where he can find the ca-certificates to validate the remote servers certificate. Otherwise we get a entry saying a untrusted connection gets used, means he encrypts but can’t verify the remote identity. In Ubuntu (Debian) inside the chroot path of postfix lies a file containing all ca-certificates, we just need to point postfix to it. The normal path is not accessable from inside the chroot. Thanks to Alain Kelder to point this out. With all that done, our server is good and enabled to send out his outgoing mail to other smtp servers using a secure transport layer. You can go even further and for example force encryption for specific servers on a per-site basis. But since thats not the scope of this article, please refer to the postfix TLS documentation for that. There you find also information how to optimise the encryption by disabling/enabling ciphers and similar.