PHPMailer has always rejected this but due to the piss-poor non-existing error handling in JMail said error was never reported until PHPMailer's exceptions started to be thrown with the 3.5.1 release. There are error handling mechanisms in place for the next release, but you should still validate that you actually have a string to pass into those methods (even better to do a quick JMailHelper::isEmailAddress($address) check) before calling them.

Michael - I agree - just an FYI this isn't my code. But this has effected quite a few sites that broke after the update. While I agree that people should be performing their own checks it just sucks that there was no additional checking in JMailer to prevent sites from breaking that have been working for some time.

They haven't been working, that's the main point. PHPMailer before 3.5.1 was returning a boolean false on addCC and addBCC (and other addXXX methods setting email addresses) in those cases but JMail never detected the error condition or reported it so from the implementor's side it all looked like it worked without issue. 3.5.1 made PHPMailer more strict about actually raising errors to the point that it exposed years of crap code in JMail that would in no possible way report an error on any method other than Send. It's better handled with the merge of #9881 but there's still a B/C break involved because the methods can now return booleans instead of only returning $this for chaining.

Whoever implemented JMail screwed up royally by changing the method signatures and returns from the parent PHPMailer class and not checking for any error conditions at all. Plain and simple. That's given the false impression that mail sending always worked when in reality invalid data was never getting added to the message nor was the invalid data being reported up the chain as such.

We are aware that JMail isn't implemented as it should but not something we can fix in Joomla 3. This would have to be redone in a new major version.

Thank you for your contribution.