According to https://github.com/PHPMailer/PHPMailer
PHPMailer 5.2 (which is compatible with PHP 5.0 - 7.0) is no longer being supported for feature updates, and will only be receiving security updates from now on. You will find the latest version of 5.2 in the 5.2-stable branch, and future versions of 5.2 will be tagged with 5.2.x version numbers, so existing Composer configs should remain working. If you’re using PHP 5.5 or later, we recommend you make the necessary changes to switch to the 6.0 release.
The 5.2 branch will not receive security updates after December 31st 2018.
There are backward incompatible changes with 6.0, it doesn’t work out of the box, I just upgraded it locally. The setFrom() error is gone, now I have ‘general error’, currently investigating what needs to be changed.