write-email-subject-line

Write High-Performing Email Subject Lines

Apply psychological principles and empirical benchmarks to write subject lines that earn opens without inflating expectations or triggering spam filters.

Why This Is Best Practice

Adopted by: Mailchimp, HubSpot, Klaviyo, and email-first brands like Morning Brew and The Hustle, whose business models depend on consistent open rates above industry average. Impact: Litmus (2024) reports that 47% of recipients decide whether to open an email based on the subject line alone; DMA (2023) shows that personalized subject lines improve open rates by 22% on average; Mailchimp data shows emoji-augmented subject lines outperform plain text by 56% in specific verticals. Why best: The subject line is the single highest-leverage word in email marketing — it gates all downstream value. Systematic approaches that combine psychological triggers (Cialdini's influence principles), deliverability hygiene, and A/B testing outperform gut-feel copywriting by 15–40% on open rate.

Sources: Cialdini "Influence: The Psychology of Persuasion" (1984, 2021 revised); Mailchimp "Email Subject Line Research" (2023); DMA "Email Benchmarks Report" (2023); Litmus "State of Email" (2024); Copyhackers subject line testing case studies (2019–2024).

Steps

1. Define the single job of this email — Before writing, answer: what do you want the reader to feel/know/do after opening? A subject line can only tease one thing. Emails with a clear single purpose have 42% higher click rates (Mailchimp, 2023). Write the job as a sentence, then compress it into the subject line.

2. Apply one primary psychological trigger — Choose one of six proven triggers: (a) Curiosity gap — "The metric your team is ignoring"; (b) Urgency/scarcity — "48 hours left: [offer]"; (c) Social proof — "12,000 teams switched to this workflow"; (d) Self-interest — "Cut your reporting time in half"; (e) Novelty — "A new way to handle churn"; (f) Fear of missing out — "What your competitors read last week." Mixing triggers dilutes impact.

3. Write 10 subject line variants — Force quantity before quality. Write all 10 before evaluating any. Variants should cover: question vs. statement, short vs. long, with vs. without number, personalized vs. generic, with vs. without emoji. Quantity expands your creative range and reveals angles you wouldn't reach after three attempts.

4. Apply the 40-character mobile test — 46% of emails are opened on mobile (Litmus, 2024). The most important words must appear in the first 40 characters. Count characters on your top candidates. If the key hook lands after character 40, rewrite front-loaded.

5. Check deliverability red flags — Remove: all-caps words, excessive punctuation (!!!), spam trigger words (free, guarantee, no obligation, winner, act now), and misleading "Re:" or "Fwd:" prefixes. Run through a deliverability checker (mail-tester.com or GlockApps) before sending to large lists.

6. Write a preview text that extends the hook — Preview text appears alongside the subject line in most inbox clients and functions as a second subject line. It must not repeat the subject line — it must continue it. If subject is "The metric your team is ignoring," preview text adds: "Most growth teams track acquisition but never touch this one number."

7. Select the two strongest variants for A/B testing — Test subject line against subject line, not subject line against preview text simultaneously. Use a statistically valid split: minimum 1,000 emails per variant, 4-hour minimum test window before declaring a winner. Track open rate, not just clicks — clicks conflate subject line performance with email content quality.

8. Send at the right time — Subject line strength interacts with send time. DMA (2023) benchmarks: Tuesday–Thursday outperform Monday/Friday by 8–12%; 10am and 2pm local time outperform other windows. Match your specific audience's behavior using your own historical data after 3+ campaigns.

9. Record winners in a subject line swipe file — Log every test: variant A, variant B, winner, open rate delta, list size, send date, industry context. After 20+ tests, patterns emerge specific to your audience (question-form subject lines outperform for B2B lists; urgency outperforms for DTC). This compounding knowledge is a durable competitive advantage.

10. Audit for authenticity — The subject line is a promise. Every word that inflates expectations reduces long-term deliverability through higher unsubscribe rates and spam reports. If your open rate is high but click rate is low, your subject line is over-promising. Align subject line tone with the email's actual content.

Rules

• Never use deceptive subject lines — they violate CAN-SPAM, GDPR, and CASL and destroy list health long-term.
• Test one variable at a time; do not change subject line and from-name simultaneously.
• Personalization (first name) improves open rate, but only if your list data is clean — incorrect names do more damage than no personalization.
• Maintain a subject line word frequency log; the same curiosity hook loses power with the same list after 3–4 uses.

Common Mistakes

• Writing clickbait that the email doesn't deliver — "You won't believe this" followed by a routine newsletter destroys trust and trains subscribers to ignore future subject lines.
• Making it too long — Subject lines over 60 characters are truncated in Gmail and most mobile clients. The hook must land before the cut. Average winning subject line length is 41–50 characters (Mailchimp, 2023).
• Ignoring the from-name — Subscribers read from-name + subject line as a unit. A strong subject line from an unrecognized sender underperforms a moderate subject line from a trusted brand name.
• Not testing on real subscribers — Preview tools (Litmus, Email on Acid) show rendering, not engagement. Actual open rates only come from sending to real humans.

Examples

Curiosity gap: "The mistake most founders make in month 3" — withholds the payoff, forcing an open.

Urgency with specificity: "Your free trial ends in 23 hours" — specific countdown outperforms "expires soon" by 18% (Klaviyo internal data, 2022).

Self-interest: "How to write a performance review in 20 minutes" — the reader immediately knows the value; no ambiguity.

When NOT to Use

• Transactional emails (order confirmations, password resets) — use clear, functional subject lines; psychological triggers are inappropriate and reduce trust.
• When you have fewer than 500 subscribers — A/B testing is statistically invalid; focus on writing one great subject line and building list size first.