I was coming across this same error on one of the sites I manage for my clients. Morandan's suggestion, via the link, helped me locate my issue.
The reason I was getting this error was due to having multiple to: emails configured incorrectly. I initially had: "email1@domain.com; email2@domain.com" in the To: line of the mail section. This is incorrect as the semicolon is not recognized by the plugin script. I changed the semicolon to a comma and my error no longer occurs.
I hope this helps others as well.