Mike:
I just received the same error and the solution is pretty easy (and stupid at the same time) sometimes the email that you are sending has an invalid email address. The error doesn't explain something like that but it's true. I had the same one but that exception is raised only with emails that are being sent to fake emails (writed by me just to perform testing).
This answer is too late for your question, but maybe it's helpful to future references.
Regards
Diego