Why “Contact Form 7” doesn’t update PHPmailer library?

PHPMailer is not part of the plugin, it is shipped with WordPress. So it is up to WordPress to keep it up to date. There is a patch already in ticket #37210.

In the mean time, you can create a mu-plugin, include the newer version of PHPMailer here, create an instance and assign it to the global variable $phpmailer. WordPress will then use that instead of its own outdated version. But be aware that WordPress’ version has some changes that are not compatible with the original class, so you might break some plugins.

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