Problem

Getting "Invalid Address" error after upgrading to Joomla 3.5.1.

Solution

Upgrade to version 1.8.7+

Discussion

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.

See: http://joomla.stackexchange.com/questions/16051/fix-the-invalid-address-error-after-upgrading-to-joomla-3-5-1

Thanks to Aaron Holte for spotting the problem + doing the research on the problem.