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
Call to Action (CTA)
- “Shop Now” vs. “Discover More”
- Button color, size, and placement
- Text-only vs. graphical buttons
Send Time & Cadence
- Behavior-based vs. fixed time
- Morning vs. afternoon
- Weekdays vs. weekends
Personalization Techniques
- Subject line only vs. full body personalization
- Dynamic content blocks based on past behavior
- Using purchase history vs. browsing activity
Use a Simple Testing Framework
- Hypothesis: What do you believe will improve performance?
Example: “A more direct CTA will increase clicks by 15%.” - Variable: Test only one element at a time.
- Subject, headline, layout, CTA, etc.
- Control vs. Variant: Keep everything else identical.
- You need consistency to isolate impact.
- Audience Split: Divide your audience randomly, but equally.
- Most ESPs allow 10/10/80 splits for testing → winner goes to 80%
- Minimum Sample Size: Avoid premature conclusions.
- Use tools like ABTestGuide or Optimizely's calculator to determine minimum recipients per version.
- Analysis & Application: Document and apply learnings.
- Track not just opens/clicks, but conversion, RPE, and retention impact over time.
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