Running into a similar redirect issue with an improper email as several others are noting on here. I did some basic tests on it, here is what I am seeing happen on my end:
1. An improper email redirects to an old blog article with #mc4wp-form-1 appended to the end of the URL.
2. I am not receiving any notification that there was an improper email submit
3. This is the same result whether I input an improper email, leave the field blank, or submit an email that is already in the Mailchimp list.
You can test this for yourself on my site at http://ofdustandkings.com