Error in Mailing Extensions
From Joomla! Documentation
Errors reported[edit]
"Invalid Address" is reported when sending emails
Versions affected[edit]
This pertains only to Joomla! version(s): 3.5.1 and higher
What is the cause[edit]
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[edit]
Update your extensions to the latest versions.
How to fix (for Extension Developers)[edit]
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.
Further Reading[edit]
This post on the forum gives additional developer information http://forum.joomla.org/viewtopic.php?f=710&t=917243#p3386316