Day 12: Email Subject Line Lab
The Concept
The subject line is the only part of your email that 100% of recipients see. Everything else — the copy, the design, the offer — is gated behind it. Yet most marketers write subject lines in 90 seconds at the end of a campaign build, when their energy is low and their deadline is close. That is the backwards approach, and it is costing you opens.
Volume is the discipline
The problem with writing one or two subject lines and picking the best one is that you are optimising within a tiny solution space. Twenty variants forces you to exhaust the obvious options quickly and then find angles you would not have considered at all. The contrarian and curiosity-gap categories in particular tend to produce subject lines that feel uncomfortable at first — and those are often the ones that perform best, precisely because they break the pattern of every other email in the inbox.
What the scoring does
The AI scoring across open rate potential, spam risk, and brand fit is not a replacement for real A/B testing data — nothing is. But it does something valuable: it forces an explicit evaluation of each variant on three dimensions rather than a gut-feel ranking. Spam risk in particular is easy to miss when you are close to the copy. Phrases like "free", "urgent", "act now", "limited time offer", and excessive capitalisation or punctuation can trigger spam filters before your email reaches the inbox. The scoring flags these proactively.
Preview text is not an afterthought
Most email clients show a preview text snippet of 40–90 characters after the subject line. This is prime real estate that the majority of email marketers waste by letting it default to "View this email in your browser" or the opening line of the email body. Today's power tip gives you 5 preview text variants specifically written to complement the subject line — adding new information, not repeating it. Subject line plus preview text together are the full opening pitch. Treat them as a unit.
Build your swipe file
Save the top 5 subject lines from every run of this prompt, even the ones you do not use. Over time you will build a swipe file of subject line structures that work for your specific audience and brand. That file becomes increasingly valuable as your team grows and briefing new copywriters becomes part of the job.
Prompt of the day
Copy this into your AI tool and replace any bracketed placeholders.
Prompt
You are an email marketing specialist with expertise in deliverability, open rates, and copywriting. I need to generate and evaluate subject line variants for an email campaign. The email is from [BRAND/SENDER NAME] to [AUDIENCE SEGMENT: e.g. lapsed customers / new subscribers / active users / prospects]. The email's purpose is [EMAIL PURPOSE: e.g. promote a limited offer / share a new blog post / announce a product update / re-engage inactive subscribers]. The core offer or news is: [DESCRIBE WHAT THE EMAIL IS ABOUT IN 1–2 SENTENCES]. The desired action is: [WHAT YOU WANT THE READER TO DO]. The tone should be [TONE: urgent/curious/warm/direct/playful]. Generate 20 subject line variants across the following 6 strategies: 1. Curiosity gap (3 variants) — create intrigue without being clickbait. 2. Direct benefit (3 variants) — state exactly what is in it for the reader. 3. Personalisation or specificity (3 variants) — use numbers, timeframes, or audience-specific language. 4. Question (3 variants) — a genuine question the reader wants answered. 5. Urgency or scarcity (3 variants) — time-sensitive or limited framing where appropriate. 6. Contrarian or surprising (5 variants) — challenge an assumption or say the unexpected. After generating all 20, score each one out of 10 across three criteria: open rate potential (intrigue + relevance), spam risk (avoid trigger words), and brand fit for a [TONE] brand. Recommend the top 3 overall and explain why.
Your 15-minute task
Pick an email you are sending in the next two weeks. Fill in the prompt with the email's details and run it. From the top 3 recommended subject lines, choose one as your primary and one as your A/B test variant. If your email platform supports A/B testing, set it up before closing the tab.
Expected win
20 scored subject line variants for a live email campaign, with a recommended primary subject line and an A/B test variant ready to deploy.
Power user tip
After the scoring, send: 'Now write 5 preview text variants (under 90 characters each) that pair with the top-rated subject line. Each preview text should add new information rather than repeat the subject line, and end with an implicit reason to open.'