If you are having trouble with not adding new subscribers to your Mailchimp list from Retail Express, look for "required" fields. These might make sense on your signup form but will prevent the information from Retail Express being able to add any new subscribers.
For example, if your signup form includes a Gender field that is required, this will cause the Mailchimp connector to get errors after we try to attempt to subscribe them as we may not have that information.
Getting Started with Merge Tags for Email: https://eepurl.com/dyilOL
To check each field's settings, navigate to the Form Builder for the audience you're working with. If you find a field marked required, it is probably the cause of your issue.
Remember the Mailchimp connector is another source of valuable marketing information and needs to be considered as well as your sign up form if you want to get the most value from our connector working away in the background.