I realized that popular Wordpress plugin
Contact Form 7 uses this library https://github.com/PHPMailer/PHPMailer.
When I check source of mail, which has been sended from Contact Form 7, it looks like this:
X-Mailer: PHPMailer 5.2.14 (https://github.com/PHPMailer/PHPMailer) X-WPCF7-Content-Type: text/plain
So, this version is from (Nov 1st 2015).
The latests one is:
Version 5.2.18 (December 24th 2016)
SECURITY Critical security update for CVE-2016-10033 please update now! Thanks to Dawid Golunski.
Why Contact Form 7 uses old versions of PHPMailer?