When the API responds with a sent status, everything works fine:
[{"email": "me@here.com", "status": "sent"}]
But when the status comes back as queued, which it often does with a long list of addresses, wpmandrill falls back to wp_mail_native. I propose that queued should be a valid status as well.
My workflow is to send these messages to a large list of people while preserverecipients is disabled, effectively making it a massively BCC'd message. However when the above scenario happens, the full list of recipients is disclosed. PROBLEM.