This is normally where the recipient of the email has reported the email as spam.

They can easily do this if their email address was Hotmail or Yahoo as there is a link in those email providers to report spam.

By reporting the email as Spam this would show as Complaint within the contacts history and also logged on the Dashboard.

The recipients have automatically been un-subscribed from receiving email from you again.

Please read our e-Marketing Anti-Spam policy to find out more.