The Mandrill Delivered-status in the UI doesn’t actually mean that it is sent, only that Mandrill have received the message for processing. This is of course extremely confusing.
The only way of seeing if an email is actually sent (i.e. successfully delivered to the receiving mail server) is to see if the message has smtp-events. Do note that it can take some time before the SMTP-events are available in the GUI/API (I have experienced a delay of between 2 minutes and 24 hours).
To see all emails that not currently confirmed delivered you can search for “NOT smtp_events.diag:250” in the search field.