How to Run A/B Tests on Email Subject Lines in Letterhead

A/B testing lets you try two subject lines against each other and automatically send the winner to most of your audience.

Before you start

• You need at least 400 audience members — below that, the A/B toggle isn’t available.
• A/B testing currently covers email subject lines only (not preview text or newsletter content).
• Letterhead splits your audience automatically: 25% receives the test (split between the two subject lines), and 75% receives the winner based on open rates.

Set up the test

1. Open your draft from Emails and continue to the Preview and Schedule step.
2. Toggle Create A/B Test.
3. Enter two subject line variations in the Email Subject fields.
4. Choose your Audience, Date, and Time (and optionally send a test email).
5. Click Schedule.

Letterhead sends the variations to 25% of your audience, measures open rates for one hour, then automatically delivers the winning subject line to the remaining 75%.

Reviewing results

After the send, the newsletter’s metrics show the A/B Test Results: both subject lines in full, a “winner” badge on the best performer, and opens, open rate, and recipients reached for each. See Newsletter Delivery and Engagement Metrics Report for the rest of your post-send data.

FAQs

• Can I test more than two subject lines? Not at this time — each test supports two variations.
• When is the winner sent? One hour after the test begins; the first hour measures open rates from the 25% test group.
• Where do I see past results? On each published newsletter’s metrics, in the A/B Test Results section.

Struggling to write two strong candidates? Smart Subjects can generate them with AI. For everything else, see Working in Emails.