reaperinvest Email Subject Line Tester
When to Use This Skill
Use this skill when you need to:
- Generate 20+ subject line variations for an email campaign
- Score subject lines against proven open rate formulas
- A/B test subject line pairs optimized for different psychological triggers
- Predict relative open rate performance before sending
DO NOT use this skill for SMS copy, push notification text, or ad headlines. This is specifically for email subject lines and preview text.
Core Principle
THE SUBJECT LINE'S ONLY JOB IS TO GET THE EMAIL OPENED — IT MUST CREATE ENOUGH CURIOSITY OR PROMISE ENOUGH VALUE THAT NOT OPENING FEELS LIKE A LOSS.
Phase 1: Brief
Gather the context needed to write relevant, targeted subject lines.
Required Inputs
| Input | What to Ask | Default |
|---|---|---|
| Email topic/offer | "What is this email about?" | No default — must be provided |
| Audience | "Who is receiving this email?" | Existing email subscribers |
| Email type | "What kind of email? (newsletter, promo, announcement, nurture)" | Promotional |
| Brand voice | "Casual, professional, playful, urgent?" | Conversational and direct |
| Any words to avoid | "Words or phrases that don't fit your brand?" | None |
GATE: Confirm the brief before generating subject lines.
Phase 2: Generate Variations
Produce 25 subject lines organized by psychological formula.
Formula Categories
Generate at least 3 subject lines per category:
1. Curiosity Gap — opens a loop the reader must close
- "The pricing mistake that cost me $4,200"
- "I stopped doing this and my revenue doubled"
2. Benefit-Driven — states the clear outcome
- "Get 3x more replies with this email template"
- "Save 5 hours this week with one automation"
3. Urgency/Scarcity — time or quantity pressure
- "Last chance: price goes up at midnight"
- "Only 12 spots left for the live workshop"
4. Question-Based — triggers mental engagement
- "Are you making this invoicing mistake?"
- "What would you do with 10 extra hours per week?"
5. Social Proof — leverages others' results
- "How Sarah went from $2K to $10K months"
- "427 solopreneurs already grabbed this"
6. Pattern Interrupt — breaks inbox scanning patterns
- "Don't open this email (seriously)"
- "I was wrong about email marketing"
7. Listicle/Number — specific and scannable
- "5 subject line tricks that doubled my open rate"
- "3 things I'd do differently starting a business"
8. Personalization — uses subscriber data
- "{first_name}, your custom growth plan is ready"
- "Quick question about your business, {first_name}"
Scoring Criteria
Score each subject line on a 1-10 scale across these factors:
| Factor | Weight | What It Measures |
|---|---|---|
| Curiosity | 25% | Does it create an open loop? |
| Clarity | 20% | Is the value clear in under 2 seconds? |
| Emotion | 20% | Does it trigger a feeling (fear, excitement, pride)? |
| Length | 15% | 6-10 words / 30-50 characters optimal for mobile |
| Specificity | 20% | Does it use concrete numbers or details? |
Phase 3: Rank and Recommend
Deliverables
- Top 5 ranked subject lines with scores and reasoning
- 3 A/B test pairs — each pair tests one variable (curiosity vs. benefit, short vs. long, question vs. statement)
- Preview text for each top 5 subject line (40-90 characters that complement, not repeat, the subject)
- Spam trigger check — flag any words that may trigger spam filters (free, guarantee, act now, etc.)
Output Format
## Top 5 Subject Lines
1. "The pricing mistake that cost me $4,200" — Score: 8.7/10
Preview: "And the 30-second fix that stopped the bleeding"
Why: Specific dollar amount + curiosity gap + loss aversion
2. [Next subject line...]
GATE: Present rankings and get user approval before finalizing.
Phase 4: Polish
1. Platform-Specific Notes
Flag any issues for common platforms:
- Gmail clips subject lines over 70 characters
- Mobile shows 30-40 characters before truncating
- Outlook may hide preview text
2. Emoji Recommendations
Suggest 2-3 subject line variants with emojis and note that emoji performance varies by audience (B2B generally lower, B2C generally higher).
3. Send Time Suggestions
Recommend optimal send times based on email type (promotional: Tuesday/Thursday 10am, nurture: morning, urgent: varies).
Anti-Patterns
- ALL CAPS subject lines — feels like shouting and triggers spam filters.
- Misleading subject lines — the email must deliver what the subject promises. Bait-and-switch kills trust.
- Over-using urgency — if every email is "LAST CHANCE," none of them are.
- Ignoring mobile length — most emails are opened on mobile. If the subject gets cut off, the hook is lost.
- Re:, Fwd: tricks — fake reply/forward prefixes are deceptive and damage sender reputation.
- No preview text — leaving preview text blank wastes prime real estate.
Recovery
- Vague email topic: Ask for the one thing the reader should do after opening. Build subject lines around that action.
- User dislikes all variations: Ask which category (curiosity, benefit, urgency) feels closest to their voice. Generate 10 more in that style.
- Spam words flagged: Provide alternative phrasing that conveys the same urgency without trigger words.
- No audience info: Default to general solopreneur audience and note that personalization improves with audience specifics.