My customer has a Drupal site and I am working my services with him through an Email Marketing reseller account. 

My customers has a DataBase with registed users in Drupal, and wants to syncronize the subscriptions. For example that, when one user check the subscription option to the newsletter in the site, automatically transfer that subscriber to the Email Marketing App via API.

And when a subscriber decides to cancel the subscription, also to be updated in Drupal and in Email Mk Platform.