Best PromptsEmail Subject Line Lab
Generate subject lines across distinct opening angles with matching preview text, plus a test plan that isolates one variable at a time.
Write and plan a test for subject lines. The email: [WHAT IT SAYS AND WHAT THE READER GETS FROM OPENING IT] Audience and their relationship to us: [e.g. "trial users on day 3" / "newsletter subscribers of 2 years"] What action the email asks for: [THE ONE ACTION] Sender name as it appears: [WHO IT COMES FROM] List size and typical open rate: [NUMBERS, or "unknown"] Voice constraints: [e.g. "never use emoji", "we are a bank"] Produce: 1. **20 subject lines**, four each across five angles, so a test measures the angle rather than the wording: - Curiosity gap: an open loop the email closes - Direct benefit: the outcome, stated plainly - Specific and concrete: a number, name, or detail - Question: one the reader would answer yes to - Plain and personal: what a colleague would type, lowercase and unstyled For each: character count, and the preview text that completes rather than repeats it. 2. **Best three**, ranked, with one line on the mechanism behind each. 3. **Deliverability check**: flag any line likely to trip spam filtering (all caps, excessive punctuation, certain money and urgency phrasings) and give a clean alternative. 4. **Promise audit**: for each of the top three, does the email body deliver what the subject line implies? Name the mismatch if there is one. A curiosity gap the email never closes buys one open and costs several later. 5. **Test plan**: what to test first, how to split, what sample size makes the result meaningful given my list size, how long to wait, and what result would be too small to act on. If my list is too small for a reliable subject line test, say so directly and suggest what to test instead. Rules: no fake urgency, no false personalization, no "Re:" or "Fwd:" on a first-contact email. The subject line is a contract with the reader.
How to use
The preview text is the half of this that most teams leave on default, and it is roughly as much visible real estate as the subject line itself. Writing it to complete the subject rather than repeat it is a free lift. Take the sample-size answer seriously: on a list of a few thousand, most subject line A/B tests cannot distinguish a real effect from noise, and acting on those results is how teams end up with a house style built on coin flips.
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