Email Subject Lines

Email Subject Lines — Sub-Skill

Overview

Subject lines determine whether your email gets opened or ignored. This sub-skill provides 30+ proven formulas, spam trigger detection, personalization strategy, and A/B testing frameworks for subject line optimization.

Command: /email subjects

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Subject Line Audit Checklist

When auditing subject lines, check:

• [ ] Length (40-50 characters for mobile optimization)
• [ ] Preview text set (40-130 characters)
• [ ] No spam trigger words
• [ ] Personalization tokens used appropriately
• [ ] Emoji usage (if any) is intentional and tested
• [ ] Matches email content (no clickbait)
• [ ] Creates curiosity or urgency without being misleading
• [ ] A/B tested before full send

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Length Optimization

| Device | Visible Characters | Recommendation | |--------|-------------------|----------------| | iPhone (Portrait) | 35-40 chars | Keep key info in first 35 chars | | Android | 40-45 chars | Front-load the value proposition | | Desktop Gmail | 70-80 chars | Can be longer but front-load anyway | | Desktop Outlook | 60 chars | Moderate length | | Apple Watch | 18-20 chars | First 20 chars must hook |

Best practice: Write for 40-50 characters. Put the most important words first. Never rely on characters after position 40 being visible.

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30+ Subject Line Formulas

Curiosity & Open Loops

1. The [thing] most [audience] get wrong about [topic]
2. I was wrong about [topic]...
3. This changed how I think about [topic]
4. The [adjective] truth about [topic]
5. What [expert/brand] doesn't want you to know about [topic]
6. Stop doing [common practice] (here's why)

Numbers & Lists

1. [Number] ways to [achieve outcome] in [timeframe]
2. [Number] [topic] mistakes costing you [negative outcome]
3. The [number]-minute [topic] fix
4. [Number]% of [audience] don't know this about [topic]

Questions

1. Are you making this [topic] mistake?
2. What would you do with [specific benefit]?
3. Ready to [achieve outcome]?
4. Is your [thing] costing you [money/time/customers]?

Urgency & Scarcity

1. [Offer] ends tonight
2. Last chance: [specific benefit]
3. Only [number] spots left for [offer]
4. [Hours/days] left to [get benefit]

Personalization

1. [First name], your [thing] is ready
2. Specifically for [segment/industry]: [benefit]
3. Based on your [action], you might like [this]
4. [Company name] + [your brand] = [outcome]

Social Proof

1. How [customer name] achieved [specific result]
2. [Number] [audience] already [did thing]
3. Why [well-known brand] switched to [approach]
4. "[Quote from customer]" — [Customer name]

Value & Benefit

1. Get [specific benefit] without [common pain point]
2. The fastest way to [achieve outcome]
3. [Outcome] in [timeframe] (no [common objection] required)
4. Save [time/money amount] on [thing] this [week/month]

Re-engagement

1. We miss you, [First name]
2. It's been a while... here's what you missed
3. Should we stop emailing you?
4. Your [thing] is waiting for you

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Spam Trigger Words to Avoid

These words and phrases increase the likelihood of landing in spam. Avoid them, especially in combination or in ALL CAPS.

Financial/Money (High Risk)

$$$, 100% free, Act now, Additional income, Affordable, Apply now, As seen on, Be your own boss, Big bucks, Billion, Bonus, Cash, Cash bonus, Casino, Cheap, Collect, Compare rates, Cost, Credit, Discount, Don't miss out, Double your, Earn, Earn extra, Easy money, Extra cash, Fast cash, Financial freedom, For free, Free access, Free consultation, Free gift, Free info, Free investment, Free membership, Free money, Free offer, Free preview, Free trial, Get out of debt, Get paid, Giveaway, Guaranteed, Income, Incredible deal, Investment, Join millions, Lifetime, Limited time, Loan, Lowest price, Lucky, Make money, Million dollars, Miracle, Money back, No catch, No cost, No fees, No obligation, No purchase necessary, No strings attached, Obligation, Offer, Once in a lifetime, One time, Pennies, Please read, Potential earnings, Prize, Promise, Pure profit, Risk free, Satisfaction guaranteed, Save big, Save up to, Special promotion, Unlimited, Urgent, While supplies last, Winner, Winning, You have been selected, You're a winner

Medical/Health (High Risk)

Cure, Diagnose, Diet, Lose weight, Medical breakthrough, Miracle cure, No prescription, Pharmacy, Weight loss

Action Pressure (Medium Risk)

Act immediately, Act now, Apply immediately, Buy now, Call free, Call now, Click below, Click here, Click now, Contact us immediately, Deal ending, Do it today, Don't delete, Don't hesitate, Exclusive deal, Expire, Get it now, Get started now, Hurry up, Immediately, Instant, Now only, Offer expires, Once in a lifetime, Order now, Please read, See for yourself, Sign up free, Special offer, Take action, This won't last, Time limited, What are you waiting for

Formatting Triggers (Medium Risk)

• ALL CAPS in subject line
• Multiple exclamation marks (!!!)
• Dollar signs ($$$)
• Excessive punctuation (???, ...!!!)
• RE: or FW: when not actually a reply/forward

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Preview Text Optimization

Preview text (preheader) is the text that appears after the subject line in the inbox. It's your second chance to get the open.

Best Practices

• Length: 40-130 characters (85-100 chars is the sweet spot)
• Don't repeat the subject line
• Extend the story — add context the subject line started
• Include a CTA if appropriate
• Never leave it blank — inbox will show first line of email body

Preview Text Formulas

| Subject Line Type | Preview Text Strategy | |-------------------|----------------------| | Question | Answer it partially | | Number/List | Tease the first item | | Curiosity | Add a second hook | | Urgency | Specify the deadline | | Personalization | Add the benefit |

Example Pairs

Subject: 5 email mistakes killing your open rate
Preview: #3 is the one most marketers make without realizing it

Subject: Your cart is waiting
Preview: Complete your order in 60 seconds — free shipping included

Subject: [First name], quick question
Preview: It'll take 30 seconds and could save you $200/month

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Personalization Tokens

| Token | Example | Impact on Open Rate | |-------|---------|-------------------| | First name | "Sarah, your report is ready" | +10-14% | | Company name | "How Acme Corp can save 30%" | +15-20% | | Location | "New for Chicago marketers" | +12-15% | | Past purchase | "Love your [product]? Try this" | +20-25% | | Behavior | "You viewed [item] — here's 10% off" | +25-35% |

Warning: Always set fallback values. Hi {first_name|there} not Hi {first_name}. Seeing "Hi {first_name}" destroys trust instantly.

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Emoji Usage in Subject Lines

When to Use

• Brand voice is casual/friendly
• B2C audiences (especially millennials/Gen Z)
• Seasonal/holiday emails
• To stand out in a crowded inbox

When to Avoid

• B2B/enterprise communications
• Legal, financial, or medical content
• Transactional emails
• If audience skews 55+

Top-Performing Emojis by Category

| Category | Emojis | Use Case | |----------|--------|----------| | Urgency | Fire, alarm, lightning | Flash sales | | Celebration | Party, confetti, star | Milestones, launches | | Seasonal | Holiday-specific | Seasonal campaigns | | Pointing | Arrow, pointing hand | Drawing attention |

Rule: Max 1-2 emojis per subject line. Place at beginning or end, never in the middle of words.

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Subject Line Scoring

Score each subject line 0-100:

| Criterion | Weight | Scoring | |-----------|--------|---------| | Length (40-50 chars) | 20% | 100 if 40-50, -10 per 5 chars outside range | | No spam triggers | 25% | -25 per trigger word found | | Personalization | 15% | 100 if personalized, 50 if generic | | Preview text present | 15% | 100 if set, 0 if blank | | A/B tested | 15% | 100 if tested, 0 if not | | Mobile-friendly | 10% | 100 if key info in first 35 chars |

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: 0-shiv
• Source: 0-shiv/secondstep-claude-skills
• License: MIT

Install and usage instructions live in the source repository linked above.