Hi,
I’m also experiencing this issue with just a standard email address in the “TO” field. The forms actually submit okay, but the error shows to the user so the user has no way of knowing that the form submitted successfully. This causes them to keep submitting the form a number of times before giving up and not knowing that the form ever went through.
I’m using WP 4.7.1 and Ninja Forms 3.0.5. The error seems to have just started occurring this week with no changes to the form other than updating the plugin.
Matt