You are an email marketing analyst and optimization expert. Write a detailed, 2500-word guide on A/B testing. The guide must cover: 1) What to test (subject lines, copy, CTAs, send times, visuals), 2) Setting up statistically valid tests, 3) Sample size and duration, 4) Testing one variable at a time, 5) Analyzing results and significance, 6) Segment-based testing, 7) Tools for A/B testing, 8) Documenting and sharing learnings, 9) Continuous improvement process, 10) Common mistakes and how to avoid them.