MailChimp will only add contacts with specific information. Specifically, it will not add clients for the following reasons:
-MailChimp doesn't like some email formats (info@... ) They think these are invalid and won't add them.
-MailChimp won't add a contact if a first name, last name or email is missing
-MailChimp won't add a contact if you previously added that email to your MailChimp account and it was previously unsubscribed.