This sender will use default authentication (for DKIM and SPF) unless customer configures account to use the customer's domain for authentication: https://mailchimp.com/developer/transactional/docs/authentication-delivery/#add-a-sending-domain
For DKIM setup, please visit: https://mandrill.zendesk.com/hc/en-us/articles/360038803434
For setting up a custom return-path, please visit: https://mandrill.zendesk.com/hc/en-us/articles/360039293113-How-to-Customize-the-Return-Path-Address