Complaint rate
Percentage of recipients who marked the email as spam — must stay under 0.1% to avoid deliverability damage.
Complaint rate is the percentage of recipients who marked the email as spam (or "junk" or "phishing"), calculated as (complaints / delivered) × 100. ISPs treat complaints as one of the strongest negative reputation signals. Most senders need to stay under 0.1% to avoid material deliverability damage; above 0.3% is in danger territory.
Spam complaints come from three main sources: subscribers who don't remember signing up (the most common cause), subscribers who couldn't easily find the unsubscribe (the structural cause), and subscribers angry about content (the content cause). The structural fix is making unsubscribe trivially easy — counterintuitively, an easier unsubscribe reduces total spam complaints because frustrated subscribers take the easier path.
Gmail's feedback loop and Microsoft's JMR (Junk Mail Reporting) program let senders see which subscribers complained. Use this to suppress complainers permanently — sending more email to someone who has already complained is a guaranteed way to make the complaint rate worse on future sends.