replace directive "smtp_use_tls" w/ "smtp_tls_security_level" (2.3+ compat) 

Use of the smtp_use_tls directive is still supported, but by default the smtpd_tls_security_level is set.

http://www.postfix.org/postconf.5.html#smtpd_tls_security_level

"This feature is available in Postfix 2.3 and later."