Getting "Invalid Address" error after upgrading to Joomla 3.5.1.
Upgrade to version 1.8.7+
Joomla Mail (which uses PHPMailer) now treats replyTo as a string, not an array. Version 1.8.7 checks the Joomla version. If 3.5.1 or above, assigns a string rather than an array to the replyTo email param.
Thanks to Aaron Holte for spotting the problem + doing the research on the problem.