When a recipient of your email campaign clicks the spam button (or its equivalent, such as "junk" button) in their email application, their email service provider (such as Gmail, Yahoo, etc) typically sends a message back to Amazon Simple Email Services (Amazon SES). This is done to let Amazon and you know that this recipient has marked your email as spam.
To notify you of this action, Soapbox Mailer marks the recipient's CampaignMember record with a check in their Complaint checkbox field. Additionally, Amazon SES will also send an email to the "from" address used in your email from their email@example.com address, providing you any of the technical information that came along with the spam complaint from the recipient's email service provider.
When you receive such an email, you should verify that the complaining recipient is not included in your future email sends. This will ensure that you have happy email recipients and fall under the good graces of Amazon SES.