Mandrill automatically tracks and records the SMTP response we receive from recipient mail servers for each email you send. Some successfully delivered emails will include a "queued" notation in the SMTP response such as
250 OK; queued as 12345. The email has still been delivered to the recipient as expected, but may require additional processing before it lands in the recipient's mailbox.
For example, most times Mandrill can send email much faster than recipient servers are able to accept or process it. In many cases, things like the time of day and overall email traffic to that ISP or recipient server can affect how quickly they're able to receive and process your email.