Spam spikes in Google Postmaster Tools on days with no email sends are complex and multi-faceted. Several factors contribute, including the delayed reporting of spam complaints, where past sends influence current metrics. Changes in user behavior, aggressive spam filtering by ISPs, and issues with list hygiene (stale email addresses or spam traps) also play a role. Technical issues like inconsistencies in sender reputation, authentication protocols (SPF, DKIM, DMARC) failures, and delayed Spam Confidence Level (SCL) adjustments can lead to spam filtering even without active sending. Additionally, the presence of compromised email accounts sending unauthorized emails, or shared IP addresses where other senders' poor practices affect your reputation can lead to spam spikes. Finally, spoofing and list bombing attacks from third parties can further exacerbate the issue. Machine learning algorithms by email providers can sometimes cope with these spikes, so analysis of feedback loops, monitoring sender reputation, and securing email systems are important.