Error in Mailing Extensions
From Joomla! Documentation
"Invalid Address" is reported when sending emails
This pertains only to Joomla! version(s): 3.5.1 and higher
What is the cause
In Joomla 3.5.1 the Core Team changed how the error handling works in Joomla 3.5.1 - to work around a bug in sending SMTP emails in Joomla 3.5.0. This means that errors that were previously ignored in Joomla Versions are now escalated to a higher level and could stop emails from being sent.
How to fix
Update your extensions to the latest versions.
How to fix (for Extension Developers)
Ensure that you are following the Joomla Mail API - it has not changed - however the error reporting has - so any changes made should be completely backwards compatible with Joomla 3.4 and lower.
Generally the highest error case has been injecting an array into JMail::addReplyTo() instead of putting in two separate parameters. It is also important to trying to catch the exceptions that are being thrown since Joomla 3.5.1.
This post on the forum gives additional developer information http://forum.joomla.org/viewtopic.php?f=710&t=917243#p3386316