See bug 603750 regarding details about the problem in dev-php/PHPMailer.
As such we ask maintainers with packages suspected to be vulnerable to verify if the package is (or have been) affected.
I am the maintainer of joomla. It is unknown whether joomla has bundled an affected version of PHPMailer. Please remove joomla from portage until further notice.
Please, no comments in the tracker. Feel free to post any questions/concerns in the bug of your package.
First fix was incomplete, see bug 603972 aka CVE-2016-10045.