A spam complaint occurs when a recipient clicks on the ‘report spam’ button in their email program. If their ISP has what's called a Feedback Loop (FBL), that action is passed back to Mandrill and recorded in your account. The recipient is then moved to the Rejection Blacklist in the account and all subsequent emails to that recipient will not be delivered while the address is active on the rejection blacklist.
Spam complaints don't occur when an ISP has filtered an email to the spam or junk folder automatically. If the ISP doesn’t provide a Feedback Loop, the complaint won't be recorded in your account but future emails from you may be sent to the spam folder from that point forward unless the recipient takes action to remove the email from the spam folder.
Mandrill also tracks direct complaints. A direct complaint occurs when a recipient contacts Mandrill's abuse department, directly reporting an email as spam. Direct complaints are considered very serious as the recipient has gone to great lengths to reach out to Mandrill and report a message as unwanted or unsolicited.