To see which fields have been formatted for your contacts login to your MailChimp account and select Lists in the top column.
Select the list where your Classy contacts are getting synched, hover over Settings and select List fields *[MERGE]* tags.
From here, you will want to make sure all fields listed are unchecked under the Required column (other than an email address). If any custom fields are selected as required, uncheck the box and click Save at the bottom of the page.
If you have any additional questions, please contact our support team here.