Email Copy

The same list. Two subject lines. A 34-point difference in open rate.

A SaaS company sends their weekly newsletter to 22,000 subscribers. In a moment of curiosity, their marketer runs an A/B test on the subject line.

Version A: "August Product Update: New Features and Improvements" Open rate: 14.2%

Version B: "The thing our power users do that most people miss" Open rate: 48.1%

Same list. Same email body. Same send time. Same product. One subject line treated the email like an announcement. The other treated it like a secret.

3,100 extra people opened that email and saw the product update — just because of the subject line. Email marketing is widely cited as the channel with the highest ROI in digital marketing (based on Litmus State of Email and DMA annual research; figures vary significantly by methodology, industry, and list quality — consult the current editions for updated benchmarks), but that ROI is completely inaccessible if nobody opens the email.

The subject line is not a label. It's a promise, a tease, a hook — and it is the most important piece of copy in any email.

Subject lines: the inbox is a competition

Every email in someone's inbox is competing for the same limited attention. Most inboxes have 20–100 unread emails. Your subject line needs to win against all of them — in the 2–3 seconds someone spends scanning the list.

What makes a subject line work:

The 6 subject line formulas:

What kills subject lines:

• All caps: "HUGE SALE HAPPENING NOW" — triggers spam filters and feels like shouting
• Excessive punctuation: "Amazing offer!!!" — spam signal
• Spam trigger words: free, guarantee, no risk, winner, urgent, act now — use with caution
• Vague promises: "Important update" — could be anything, implies nothing interesting
• Long: Most inboxes show 40–60 characters on mobile. Keep it under 50 characters.

Preview text: The greyed-out text that appears after the subject line in most email clients. It's a second subject line — use it. Don't let the email client default to "View in browser" or the first line of your email.

Subject line: "The thing our power users do that most people miss" Preview text: "It's not a feature. It's a habit that takes 5 minutes a day."

Using AI for subject lines: Prompt: "Write 15 subject line variations for this email: [summarise the email content]. Use these formulas: curiosity gap, number, question, personal tone, direct, urgency. Keep each under 50 characters. Also write a preview text for each one." Generate 15, pick your top 3, test them.

Email body: the one-idea rule

Once the email is opened, most copywriters make the same mistake: they try to say everything.

The golden rule of email copy: one email, one idea, one CTA.

Emails that cover three topics get a third of the engagement of emails that commit fully to one. The reader's attention is scarce. Every link you add after the first one reduces clicks on the first one. Every idea you add after the first one dilutes the first one.

Pick your one idea. Cut everything else.

The structure of an email that gets clicked:

Length: Shorter is almost always better for marketing emails. The goal is the click, not the read. Get to the point, bridge to the CTA, and get out. If you need to share a lot of information — put it in the article/page you're linking to.

Exception: newsletters with multiple stories or digest formats, where the email IS the content. Even there, short intro paragraphs per story work better than essays.

Writing the email body

Treat it like a text message from a knowledgeable friend.

The most effective marketing emails don't sound like marketing emails. They sound like they're from a real person who knows something useful and is sharing it because they think you'll find it valuable.

Compare:

Corporate:

Human:

Same information. Completely different feel. One person skims and archives. The other person actually clicks.

The techniques:

Using AI for email body: Paste your notes or rough points into Claude and prompt: "Write an email body from these notes. Tone: conversational, like a knowledgeable friend texting someone they respect. Short paragraphs (1–2 sentences each). One idea, one CTA. No corporate language. Email should be under 200 words." Then edit the output for your own voice.

Transactional vs. marketing emails

Not all marketing emails are newsletters or promotions. Transactional emails — order confirmations, password resets, receipts, shipping notifications — have historically reported the highest open rates of any email type (50–80% (upper end of historical pre-2021 ranges; figures vary widely by email type and platform), noting that figures since Apple Mail Privacy Protection launched in 2021 are inflated by pixel pre-loading — treat as directional), because they contain information the customer needs.

This makes them prime real estate for marketing copy — but most companies waste them with bare-bones templates.

The welcome email is the most important email you'll ever send. It's opened by more people (50–70% open rate (pre-2021 benchmark estimates; higher-quality sources reported up to 80% for small, highly engaged lists — treat as directional) — treat as directional, as Apple Mail Privacy Protection since 2021 inflates open rate figures industry-wide) than any other email in your sequence. It sets the tone for the entire relationship. Yet most welcome emails are: "Thanks for signing up. Here's your free guide. Talk soon."

A great welcome email: delivers the lead magnet immediately, tells the reader what to expect, makes a low-friction first CTA (read one post, answer one question), and feels personal.

Back to the two subject lines

Version A — "August Product Update: New Features and Improvements" — told the reader exactly what was inside and gave them no reason to open it. If they weren't already curious about the product update, they had nothing to gain. Version B — "The thing our power users do that most people miss" — created a curiosity gap: it implied that some people already know something valuable, and the reader might not be one of them. That asymmetry is what drove a 34-point lift in open rate. The specificity of "power users" also did quiet work — it made the email feel like insider knowledge rather than a broadcast. The body content was identical in both versions, which proves the point cleanly: the subject line is not a label for what's inside. It's a reason to open. Write it last, treat it as its own piece of copy, and test it as aggressively as you'd test any ad.

Key takeaways

• The subject line is 80% of the email. If it doesn't get opened, nothing else matters. Use curiosity, specificity, or personality — avoid vagueness and spam triggers.
• One email, one idea, one CTA. Emails that try to cover everything get ignored. Commit to a single idea and a single action.
• Write like a knowledgeable friend, not a corporate broadcast. Short paragraphs, second person, specific details, conversational openers.
• Transactional emails generate significantly higher open rates than marketing emails (commonly cited around 3×, though reported rates are inflated by Apple Mail Privacy Protection since 2021) — they're the most underused copywriting opportunity in most businesses.
• The welcome email is the most important email you'll ever write. Treat it accordingly.