Most marketers test subject lines — and stop there. But in 2025, if you want real performance lift, your testing strategy needs to evolve.

Done right, A/B testing gives you predictable, compounding gains across open rates, click-throughs, and conversion.

What You Should Be Testing (Beyond Subject Lines)

Email Structure

• Long-form vs. short-form
• Hero image vs. headline-first
• One CTA vs. multiple links

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Call to Action (CTA)

• “Shop Now” vs. “Discover More”
• Button color, size, and placement
• Text-only vs. graphical buttons

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Send Time & Cadence

• Behavior-based vs. fixed time
• Morning vs. afternoon
• Weekdays vs. weekends

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Personalization Techniques

• Subject line only vs. full body personalization
• Dynamic content blocks based on past behavior
• Using purchase history vs. browsing activity

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Use a Simple Testing Framework

1. Hypothesis: What do you believe will improve performance?
   Example: “A more direct CTA will increase clicks by 15%.”
2. Variable: Test only one element at a time.
   • Subject, headline, layout, CTA, etc.
3. Control vs. Variant: Keep everything else identical.
   • You need consistency to isolate impact.
4. Audience Split: Divide your audience randomly, but equally.
   • Most ESPs allow 10/10/80 splits for testing → winner goes to 80%
5. Minimum Sample Size: Avoid premature conclusions.
   • Use tools like ABTestGuide or Optimizely's calculator to determine minimum recipients per version.
6. Analysis & Application: Document and apply learnings.
   • Track not just opens/clicks, but conversion, RPE, and retention impact over time.

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Don’t Test Just to Test

Testing works best when it’s part of a systematic optimization strategy, not random guessing.

Focus on:

• Bottlenecks in your funnel (e.g., high open rate, low click)
• Moments of decision-making (e.g., CTA or post-click UX)
• Segments with enough volume to produce reliable results

Explore Next: How to Run A/B Tests That Actually Lead to Lift