A sudden surge in Gmail spam filtering can arise from multiple interconnected factors. Gmail's continuously evolving algorithms, designed to combat new spam tactics, play a significant role, potentially impacting previously compliant senders. User-reported spam and phishing attempts trigger these algorithm changes. Reduced email reputation due to poor list hygiene (sending to unengaged addresses or using purchased lists), problematic content, or low engagement (opens, clicks) can increase filtering. Technical issues like incorrect DKIM setup, changes in DMARC policies, or being on a blocklisted IP/domain contribute. Spikes in email volume and temporary glitches in Gmail's infrastructure also play a part. Using shared IPs is another cause.