[Postfix] Set compatibility_level to 2

data/conf/postfix/main.cf

@@ -139,3 +139,4 @@ mail_name = Postcow
 transport_maps = pcre:/opt/postfix/conf/local_transport, proxy:mysql:/opt/postfix/conf/sql/mysql_transport_maps.cf
 smtp_sasl_auth_soft_bounce = no
 postscreen_discard_ehlo_keywords = silent-discard, dsn
+compatibility_level = 2