add variable for smtpd_banner

defaults/main.yml

@@ -3,3 +3,9 @@ postfix_relay_sasl_host: smtp.gmail.com
 postfix_relay_sasl_user: user
 postfix_relay_sasl_domain: gmail.com
 postfix_relay_sasl_passwd: secure
+
+# smtpd_banner contains $myhostname by default and
+# can't be removed
+postfix_relay_smtpd_banner:
+  - ESMTP
+  - $mail_name

templates/etc/postfix/main.cf.j2

@@ -580,7 +580,7 @@ alias_database = hash:/etc/aliases
 # You MUST specify $myhostname at the start of the text. That is an
 # RFC requirement. Postfix itself does not care.
 #
-#smtpd_banner = $myhostname ESMTP $mail_name
+smtpd_banner = {{ (['$myhostname'] + postfix_relay_smtpd_banner) | join(' ') }}
 #smtpd_banner = $myhostname ESMTP $mail_name ($mail_version)
 
 # PARALLEL DELIVERY TO THE SAME DESTINATION