Hi,
scroll down on page you have linked.
If you got the error : "> mail: cannot send message: Process exited with a non-zero status> ", you must to activate the less secure apps on you google account:
fixed by activating a "> less secure> " flag inside gmail, visiting this link:
and please read: Add Gmail to another email client - Gmail Help
Tested and works here.
X-Received: by 10.194.192.8 with SMTP id hc8mr30856575wjc.87.1463411034098; Mon, 16 May 2016 08:03:54 -0700 (PDT)
Message-Id: <5739e159.821b1c0a.9bbe7.ffffdf3a@mx.google.com>
X-Google-Original-From: "root" <root@RPi-B-Plus>
X-Mailer: mail (GNU Mailutils 2.99.98)