This error is probably not directly associated with WP Email Capture, but I thought you might have an idea of what's wrong.
I have my domain's email managed by Google Apps. I can sign up accounts within my domain but get the "Email unable to be sent" message for any other domains—including other Gmail accounts!
The temp e-mail bin recognizes the presence of "unconfirmed" email addresses, but I can't access these to confirm them. I can only delete them.
Any ideas why I am limited to being able to register ONLY other email accounts on my own domain?