@hchouhan, thanks for the response.
The only fields required by Mailchimp are first, and last name, and email address. I have now changed my form to require a separate field for first and last name, but still no luck.
There is a Warning showing in Mailchimp for WP:
[2017-03-12 11:17:12] WARNING: Contact Form 7 > No MailChimp lists were selected
