How to Write Email Subject Lines: A Step-by-Step Guide
Your subject line is the single most important factor in whether someone opens your email. This guide covers the art and science of writing subject lines that get opened — with formulas, examples, and testing strategies.
Step-by-Step Instructions
Keep it under 50 characters
Mobile devices truncate subject lines beyond 50 characters, and over 60% of emails are opened on mobile. Get your core message across in the first 40-50 characters. This constraint forces you to be specific and choose every word carefully. 'The 3 metrics that predict newsletter success' (46 characters) works perfectly.
Use specificity and numbers
Specific subject lines outperform vague ones. '7 tools that saved me 10 hours/week' is more compelling than 'Useful productivity tools'. Numbers add credibility and set clear expectations about what the reader will find inside. Odd numbers tend to perform slightly better than even ones in testing.
Create curiosity without clickbait
The best subject lines create an information gap that the reader wants to close. 'The inbox metric everyone ignores (and shouldn't)' makes you want to know which metric. But your content must deliver — clickbait that doesn't pay off destroys trust and increases unsubscribes. The subject line is a promise; the content is the delivery.
Test different approaches
Develop a repertoire of subject line styles and rotate between them: question-based ('Are your emails landing in spam?'), how-to ('How to double your open rate in 30 days'), listicle ('5 newsletter trends for 2026'), news ('Gmail just changed everything for senders'), and direct benefit ('Get more subscribers without paid ads'). Variety prevents subscriber fatigue.
A/B test systematically
Test two subject line variants on 20-30% of your list, wait 2-3 hours, then send the winner to the rest. Test one variable at a time: length, personalisation, emoji usage, question vs statement, number inclusion. Document your results to build a knowledge base of what works for your specific audience.
Avoid spam triggers
Certain patterns increase your spam score: ALL CAPS, excessive punctuation (!!!), spam trigger words ('FREE', 'Act now', 'Limited time'), misleading 'Re:' or 'Fwd:' prefixes, and emoji overuse. Keep your subject lines clean, honest, and professional. Your content quality should do the selling, not manipulation tactics.
Pro Tips
- Write 5-10 subject line options for each issue and pick the best two for A/B testing
- Study subject lines from newsletters you consistently open — what patterns do you notice?
- Front-load the most important words — they're what subscribers see first on mobile
- Use preheader text to complement (not repeat) the subject line
- Personalisation with the subscriber's name can boost open rates by 10-20%, but don't overdo it
Common Mistakes to Avoid
- Using the same formula every time — subscribers learn to tune out predictable patterns
- Making promises the content doesn't deliver — this is the fastest way to lose trust
- Using clickbait tactics that work short-term but increase unsubscribes
- Not A/B testing — you're guessing when you could be measuring
- Ignoring preheader text — it's free extra real estate to convince someone to open
How Aldus Makes This Easier
Aldus generates and A/B tests subject lines automatically on every send. The AI creates multiple variants using different styles — curiosity-driven, data-specific, question-based — and tests them on a portion of your list before sending the winner to everyone. Over time, the AI learns what resonates with your specific audience.
Frequently Asked Questions
Do emojis in subject lines help or hurt?
It depends on your audience. Emojis can increase open rates for casual, consumer newsletters but may hurt for professional B2B content. Test with your specific audience — run an A/B test with emoji vs no emoji and measure the difference.
Should I personalise subject lines with the subscriber's name?
Personalised subject lines can boost open rates by 10-20%. However, ensure your data is accurate — nothing breaks trust faster than a wrong name or an empty merge tag. Test personalisation against generic subject lines to confirm it works for your audience.
How many subject lines should I A/B test?
Test two variants (A/B) for most sends. Testing more variants requires a larger list for statistical significance — you need at least 1,000 subscribers per variant for reliable results.