Opinions about subject lines and layouts are unreliable; tests turn them into evidence and compound small gains over time.
Mistake 1: Testing several changes at once#
Why it hurts: this undermines email A/B testing at the point where mailbox providers make their decision, and it is rarely surfaced by your own tooling until placement drops.
Fix: Choose one variable per test and one primary metric (clicks or revenue, not opens).
Mistake 2: Declaring winners on open rate after Mail Privacy Protection#
Why it hurts: this undermines email A/B testing at the point where mailbox providers make their decision, and it is rarely surfaced by your own tooling until placement drops.
Fix: Size samples so results are statistically meaningful; small lists need bigger splits.
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.