Hey David,
Well, the thing is -> AcyMailing doesn't use the standard JMailer class - they have their own implementation. Because of that you are not sending the emails from acymailing through the plugin, but through their own class. Which is then in terms taking the configuration settings from your global configuration. (there you have most probably provided the smtp details to mandrill, correct?)

And the error that you have is also pointing to an smtp error. So -> make sure that your smtp settings are correct and if they are and you still have problems I would contact the mandrill support team for assistance.

Regards,
Daniel