A class that uses a few other classes to generate an email
I find it weird that you wrote this as a module rather than as a class.

How is this code going to be called?  Mail.new(user_id).send, I assume?  It doesn't seem to make much sense to call #send twice on the same object.  So, really, there isn't any point to make an object at all.  You could just as well define a function.

def send_mail_to_user(user_id)
  user = User.find(user_id)
  client = user.mailchimp
  campaign_id = client.create_campaign(user.detail.create_campaign_params)
  client.create_campaign_content(campaign_id, user.detail.create_content_params)
  client.send_campaign(campaign_id)
end