Note that Plesk doesn't ship Postfix it only downloads it from original OS vendor repo and it is quite likely it may be outdated there (especially for older OS). I see, for example:
# rpm -qi postfix
Name : postfix
Epoch : 2
Version : 2.10.1
Release : 6.el7
Architecture: x86_64
Install Date: Tue 14 Aug 2018 02:48:40 AM NOVT
Group : System Environment/Daemons
Size : 12773475
License : IBM and GPLv2+
Signature : RSA/SHA256, Fri 04 Jul 2014 11:33:15 AM NOVT, Key ID 24c6a8a7f4a80eb5
Source RPM : postfix-2.10.1-6.el7.src.rpm
Build Date : Tue 10 Jun 2014 08:39:36 AM NOVT
Build Host : worker1.bsys.centos.org
Relocations : (not relocatable)
Packager : CentOS BuildSystem <My View - CentOS Bug Tracker>
Vendor : CentOS
URL :
The Postfix Home Page
Summary : Postfix Mail Transport Agent
Description : Postfix is a Mail Transport Agent (MTA), supporting LDAP, SMTP AUTH (SASL), TLS