Recently, my provider had updated the server my webspace is hosted on to PHP 5 and while at first I didn’t notice anything wrong, there actually was a creeping background problem: all WordPress installations refused to send out any emails and in some cases even timeout error messages appeared from the PHPMailer module.

After some unsuccessful digging, I found a post on the bugtracker of PHPMailer regarding an incompatibility with PHP 5.2 – apparently there is a problem with this particular PHP version related to the Regex function. So if you suddenly have those error messages, like a timeout in class-phpmailer.php on line 879 in the case of WordPress, check what PHP version your webserver is running. In my case, the server defaults to 5.2, but I was able to switch it with the line AddHandler php55-cgi .php in the .htaccess file to 5.5 and then everything worked again.

If your webhost supports this method of switching to different PHP versions, this will fix the problem – if not, you have to ask your webspace hoster to switch versions or install a higher version yourself. Reportedly anything above 5.3 works, but I only tested it with 5.5 and it works fine with WordPress 3.9.2 and the release candidate of 4.0.