Reference:
http://wiki.zimbra.com/index.php?title=Outgoing_SMTP_Authentication#Enable_TLS
ssh root@server.mydomain.com
su - zimbra
zmlocalconfig -e postfix_smtp_tls_security_level=may
zmcontrol restart
Sent an email to normal postfix server:
Check the headers of received email:
Received: from server.mydomain.com (static.66.240.40.188.clients.your-server.de [188.40.240.66])
by mail.mydomain.com (Postfix) with ESMTPS id E457F3E09E for
Fri, 19 Apr 2013 23:35:31 +0200 (CEST)
Sent to GMAIL server:
Received: from server.mydomain.com (static.66.240.40.188.clients.your-server.de. [188.40.240.66])
by mx.google.com with ESMTPS id b4si25422716eef.188.2013.04.19.14.39.14
(version=TLSv1.2 cipher=ECDHE-RSA-RC4-SHA bits=128/128);