What does "Spam Reporting Address" mean?

An email could not be sent to John Doe 
Reason: Spam Reporting Address

I received this message when sending out a course update.  What does it mean?

If a person whom has received your email has reported that received email as spam, the email server of the recipient has marked this email as spam.  We are then notified by the ISP and we place the email that originated the spam report on a blocked email list.

Any email on this list will not be delivered by Litmos in any future mailings.

If you believe this person marked the email as spam by mistake please email support@litmos.com with the email address of the user and we will remove it from the blocked emails list.  In addition, make sure the email address system@litmos.com has been white listed to prevent having it marked as spam moving forward.