What email A/B testing is#
A/B testing sends two or more variants of a message to random samples and measures which performs better on a chosen metric before sending the winner to the rest.
Why it matters#
Opinions about subject lines and layouts are unreliable; tests turn them into evidence and compound small gains over time.
How mailbox providers use it#
Gmail, Microsoft, and Yahoo combine authentication results, sender reputation, and recipient engagement into a placement decision made per message. Email A/B testing feeds directly into that model, and weaknesses compound with other signals.
How to measure the impact#
- Baseline inbox placement with seed tests before any change.
- Make one change at a time and hold volume steady.
- Re-test after 48 to 72 hours; provider models need time to update.
- Track Postmaster Tools and SNDS alongside your seed results.
Improving it#
- Choose one variable per test and one primary metric (clicks or revenue, not opens).
- Size samples so results are statistically meaningful; small lists need bigger splits.
- Run long enough to capture late openers, typically 4 to 24 hours.
- Log results so learnings persist beyond one campaign.
Frequently asked questions#
What should I A/B test first in email?
Subject lines, then the primary call to action, then send time. These have the largest measurable effect.