quickly check your postfix mail server version. Basically, postfix is a open-source mail transfer agent (MTA) that routes and delivers electronic mail. Postfix is an alternative for Sendmail service which provide the same function.
Please execute the following command :
postconf -d | grep mail_version
Examples :
[root@centos62 ~]# postconf -d | grep mail_version mail_version = 2.6.6 milter_macro_v = $mail_name $mail_version