Email Copywriting

The subject line that tripled revenue — in one A/B test

The following is a composite scenario based on patterns common in email A/B testing. Specific figures are illustrative.

In 2019, a health supplements brand was preparing a product launch email to 85,000 subscribers. Their email team prepared two subject line options:

Version A: "Introducing our new magnesium supplement — available now" Version B: "Why 68% of adults are deficient in this mineral (and how to fix it in 7 days)"

Version B generated 3.1× more opens. That translated — because of the email platform's revenue tracking — to £47,000 more revenue from a single send.

The email content was identical. The only difference: 12 words.

Subject lines are not labels. They are promises, questions, and decisions. And they matter more than almost any other element in email marketing.

The anatomy of an email

Before writing anything, understand what your subscriber sees:

The inbox view determines whether the email gets opened. The email body determines what happens after it's opened. These are two entirely separate copywriting challenges.

Subject lines: the most important 50 characters in email

The subject line has one job: get the email opened.

Subject line formulas that consistently work:

The curiosity gap: "Why [common thing] is actually [unexpected outcome]" "The mistake 90% of [role] makes with [topic]" "I was wrong about [topic]"

Creates an information gap the reader has to open to fill. Use it when you have a genuine surprise or reversal inside.

The specific benefit: "How to [achieve outcome] in [timeframe]" "[Number] ways to [specific result]" "The [adjective] [thing] that [benefit]"

Direct and unambiguous. The reader knows exactly what they'll get. Best for tutorial or value-delivery emails.

The personal/story hook: "I made a £40,000 mistake last year" "The conversation that changed everything" "Something I've never told my subscribers before"

Signals human, not corporate. Works when the story inside is actually interesting. Overused when the email doesn't deliver on the implied intimacy.

The question: "Are you making these 5 SEO mistakes?" "What would you do with an extra 10 hours per week?" "Have you tried this yet?"

Creates a self-referential moment — the reader wonders "do I?" or "would I?" and opens to find out.

The urgency/scarcity: "Last chance: doors close tonight" "48 hours left at this price" "This disappears tomorrow"

Effective for genuine deadlines. Destructive if overused — subscribers become desensitised. Only use when the deadline is real.

Subject line rules:

Using AI for subject line generation: "Write 15 subject line options for an email about [topic/offer]. Include: 3 curiosity-gap lines, 3 specific-benefit lines, 3 story/personal lines, 3 question lines, 3 short punchy options (under 30 characters). The email content is: [brief description]. Target audience: [persona]."

Preview text: the hidden subject line extension

Preview text (also called preheader text) is the snippet of text that appears after the subject line in most email clients. It's one of the most underused elements in email marketing.

What subscribers see in their inbox:

From: Marcus Financial
Subject: The mistake I made with my own pension ↗
Preview: Here's what I wish someone had told me at 32 — and the three things I changed this year

Preview text is a second subject line. It should:

• Extend or complement the subject line (not repeat it)
• Create additional curiosity or provide the "so what"
• Be 40–130 characters (longer is truncated on most clients)

The mistake: Many marketers leave preview text blank, which causes email clients to pull the first text in the email body — often something like "View this email in your browser" or "Unsubscribe." That's a wasted opportunity and looks unprofessional.

Writing the email body

Once the email is opened, the body has one job: deliver what the subject line promised, and move the reader toward the action you want them to take.

The structure of a high-converting email:

Opening line: The first sentence must earn the next one. No preamble ("I hope this email finds you well"), no lengthy context. Start with the point, the story, or the problem.

Strong: "Your landing page is probably losing 60% of visitors in the first 8 seconds." Weak: "Hi, I wanted to reach out today to share some thoughts on landing pages."

Body copy principles:

• Short paragraphs — 1–3 sentences. Long blocks of text look overwhelming in an email.
• Write conversationally — as if writing to a specific person, not a list. Use "you" more than "our subscribers" or "our community."
• One idea per email — not three ideas, not a newsletter roundup of 12 topics. One thing, explained well, with one call to action. (Note: newsletters are different — see the Newsletters module.)
• Be specific — specifics create credibility and make abstract ideas tangible. "This changed the way I work" is weak. "I now spend 45 minutes every morning on this and it's added £3,200 a month to my revenue" is strong.

Formatting for scannability: Many email readers scan before they commit to reading. Structure to support this:

• Use bold for the most important sentence in each section
• Use short bullets when listing items (no more than 5)
• Use subheadings only in longer emails (newsletters, teaching emails)
• Add whitespace — emails with breathing room get more engagement than walls of text

CTAs: the action you're driving toward

Every email should have one primary call to action — one thing you want the reader to do after reading.

Types of CTAs in email:

• Click a link to a blog post, product, or landing page
• Reply to the email (high-trust, conversational action)
• Download something
• Register for an event
• Make a purchase

CTA placement:

• The first CTA should appear before the reader has to scroll (within the first 400 pixels)
• A second CTA at the end of the email catches readers who read through
• For promotional emails: a button. For conversational emails: a hyperlinked text link looks more human.

CTA copy:

Weak: "Click here" Weak: "Learn more" Strong: "Read the case study" Strong: "Get the template" Strong: "Book your free 20-minute call"

The button copy should describe what happens when they click, not just instruct them to click.

There Are No Dumb Questions

"How long should emails be?"

Long enough to complete the thought, short enough to hold attention. For a transactional or promotional email: 150–300 words is typically right. For a newsletter or teaching email: 400–800 words. Long-form story emails (1,000+ words) work for engaged lists where readers expect depth. The wrong question is "how many words should this be?" The right question is "what does this email need to contain to move the reader to action?" Write until you've said what needs saying, then stop.

"Should I use images in emails?"

Sparingly. Many email clients block images by default, so emails that rely on images to communicate their message are broken for a significant portion of readers. Use images only to illustrate something text can't convey (a product photo, a chart, a before/after). Avoid decorative header images — they add load time and reduce text-to-image ratio, which spam filters penalise. The best marketing emails are almost entirely text.

The email types: matching format to goal

Different emails serve different purposes — and the format, tone, and length should match:

Promotional emails: Goal: drive a specific action (purchase, registration, download) Structure: strong subject → clear benefit statement → supporting detail → prominent CTA → secondary CTA Tone: direct, benefit-focused, light on narrative

Newsletter emails: Goal: deliver value, build relationship, stay top-of-mind Structure: varies (covered in depth in the Newsletters module) Tone: conversational, opinionated, human

Teaching/educational emails: Goal: deliver specific knowledge and position you as an expert Structure: problem statement → explanation → concrete example → application → CTA (often: "reply with your question" or "here's more on this topic") Tone: expert but accessible, specific, practical

Announcement emails: Goal: communicate a change, launch, or event Structure: the news first → why it matters to them → what to do next Tone: clear and direct; don't bury the announcement

Story emails: Goal: build emotional connection, humanise the brand, often precede a sale Structure: narrative arc → lesson → bridge to relevant offer Tone: personal, vulnerable, genuine

Back to the health supplements brand

The winning subject line — "Why 68% of adults are deficient in this mineral (and how to fix it in 7 days)" — made the reader feel something before they clicked. It made them wonder if they were one of the 68%. It created a specific, personal worry with a specific, achievable resolution. The losing subject line just described a product: "Introducing our new magnesium supplement — available now." Same email. Same offer. Same price. The only difference was twelve words, and those twelve words were worth £47,000 in a single send. The email that earns the open earns the revenue. Everything else is secondary.

Key takeaways

• Subject lines determine open rate — everything else determines what happens after. Write 10 subject line options per email; send the two strongest as an A/B test.
• Preview text is a second subject line. It should extend, not repeat, the subject line. Never leave it blank.
• One email, one idea, one CTA. Emails with multiple competing calls to action perform worse than emails with a single clear direction.
• Write conversationally, not corporately. "You" over "our subscribers." Short paragraphs. Sentences that sound like they came from a real person.
• Specifics build credibility. Specific numbers, specific outcomes, specific stories outperform vague claims in every email format.