Google Postmaster Tools calculates spam complaints using feedback from Gmail users, dividing the number of spam complaints by the number of emails delivered to inboxes to determine the spam rate. This rate, along with sender reputation, authentication, and sending volume, are key factors in determining email placement and deliverability. The tools aggregate and analyze user feedback, DMARC volume, and sending patterns to provide insights into sender reputation and identify campaigns generating high spam rates. It's important to compare spam rates with DMARC volume, monitor for sudden spikes, and address high complaint rates, especially on automated emails, as they may indicate list-bombing attacks. Google attributes spam reports to when the email was received, which may not be the same day it was sent. High complaint rates negatively influence sender reputation and can trigger spam filters.