How can testing tools optimize subject lines?

Subject line testing includes: spam word checking (terms that trigger filters), length analysis (optimal for mobile truncation), preview text interaction (how subject and preview display together), and A/B testing infrastructure.

Some tools provide: performance prediction based on historical data patterns, sentiment analysis, urgency scoring, and comparison against benchmarks. These indicate likely effectiveness before sending.

Limitations: predictions based on historical patterns may not apply to your audience. Use tool suggestions as input alongside your own testing and audience knowledge. The best subject line optimization comes from systematic A/B testing with your actual recipients.