Email A/B testing is the fastest path to continuous email performance improvement, and the most misused. Brands run one subject line test, declare their winning formula, and stop testing. The brands compounding email revenue run systematic tests continuously, across subject lines, content, offers, timing, and flow logic. Here is the complete methodology.
What to Test in What Order
Test by expected impact multiplied by implementation ease. The highest-impact, easiest tests: subject lines (run on every campaign, requires no design work, produces results in 4 to 6 hours). Second tier: preview text (works in combination with subject line, affects open rate), from name (sender name matters, "Sarah from [Brand]" vs "[Brand] Team" is a testable variable), and send time (Tuesday 9 AM versus Thursday 7 PM).
Third tier (higher implementation effort, higher impact): offer structure (10 percent discount versus free shipping versus free gift), email content angle (social proof-led versus benefit-led versus urgency-led), and CTA button copy and colour. Fourth tier: full template redesign, personalisation elements, and dynamic content blocks.
Subject Line Testing Protocol
Run A/B subject line tests on 100 percent of campaign sends above 2,000 subscribers. Klaviyo's built-in test does the statistical work. What to test: length (short 4 to 6 words versus medium 8 to 12 words), format (question versus statement versus number-led), urgency language (present versus absent), and personalisation (with first name versus without). Over 12 months of consistent testing, you will accumulate 24 to 52 data points about what your specific audience responds to. This is proprietary insight that no competitor has about your list.
Flow A/B Testing: Where the Real Money Is
Campaign testing improves individual sends. Flow testing improves every future send in that flow. An abandoned cart email that is 20 percent better converting due to a flow test generates that 20 percent improvement on every abandoned cart for the rest of the time that flow runs. Compounded over 12 months, a single successful flow test creates far more revenue than a single successful campaign test.
Priority flow tests: abandoned cart email 1 send time (1 hour versus 2 hours versus 4 hours), abandoned cart email 3 offer structure (discount versus free shipping versus no offer in email 1 to 2 then offer only in email 3), welcome series email 2 content (brand story versus social proof heavy), and post-purchase cross-sell timing (day 14 versus day 21). Each of these tests has a clear hypothesis, a measurable outcome metric, and a compounding benefit when the winner is implemented.