What makes an effective email subject line?

Subject lines affect deliverability primarily through engagement impact, not filter triggers. A compelling subject line drives opens; opens signal engagement; engagement builds reputation. A boring or misleading subject line kills opens or triggers complaints—both hurt deliverability.

Direct filtering effects are minimal for legitimate senders. Modern filters don't block emails for saying "Free" or "Act Now"—they evaluate subject lines in context of sender reputation, content, and recipient behavior. However, deceptive subjects ("RE: Our conversation" when there was none) can trigger both filters and complaints.

Write subject lines for recipients, not filters. Focus on accurately representing content, compelling recipients to open, and building consistent expectations. If recipients regularly find your subject lines honest and valuable, they'll keep opening—which is the deliverability strategy. Test subject lines for performance, not spam score. The goal is engagement, and engagement protects deliverability.