[Postfix] Do not allow DSN for postscreen

data/conf/postfix/main.cf

@@ -140,3 +140,4 @@ smtp_header_checks = pcre:/opt/postfix/conf/anonymize_headers.pcre
 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