Practice
Recently Gmail started warning, verbatim, that the sender's server example.com did not encrypt this message, and shows an open padlock when viewing the message (more details in Google's help). The wording in the help center itself confused me for a long time: If you see a red broken lock icon  on a message you received or a draft, it means the message is not encrypted.. All this actually means is that the message was not transmitted over an encrypted protocol — this padlock has nothing to do with encrypting the message itself. For a long time this wording threw me off, and I was "googling" in the wrong direction until a more experienced colleague set me straight.
Since even after I figured out what the warning actually meant, I still couldn't find anything on the topic, this post is more of an explanation for people like me who didn't understand it at first. So please don't judge too harshly.
Now, to the point: to remove the warning, those who use postfix to send mail just need to add the following to/etc/postfix/main.cf
smtp_use_tls = yes smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache # this line is probably needed too smtp_tls_mandatory_protocols = !SSLv2, !SSLv3 As a result, when viewing the message source, the header that used to look like this:
Received: from smtp.279.ru (smtp.279.ru. [77.220.185.16])
by mx.google.com with ESMTP ido79si14839747lfi.52.2016.02.15.04.15.43
for <deryabinsergey@gmail.com>;
Mon, 15 Feb 2016 04:15:43 -0800 (PST)
becomes this:
Received: from smtp.279.ru (smtp.279.ru. [77.220.185.16])
by mx.google.com with ESMTPS idd124si14810044lfg.170.2016.02.15.04.20.45
for <deryabinsergey@gmail.com>
(version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Mon, 15 Feb 2016 04:20:45 -0800 (PST)
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