Your subject line has one job: to earn the open from the right person. It competes with dozens of other messages, it is often cut short on a phone screen, and it has to be honest about what is inside. Get it right and everything else in your email gets a chance. Get it wrong and nothing else matters.
There are thousands of "best subject line" lists online. Most offer formulas without context. This guide is different: it gives you a framework for writing subject lines that match your audience and goal, examples by email type, the legal line you cannot cross, and a sensible way to test now that open rates are less reliable than they used to be.
Key Takeaways
- Lead with the benefit or the specific topic; put the key words first because mobile inboxes truncate.
- Treat the sender name, subject line and preview text as one unit.
- Never mislead: the FTC's CAN-SPAM guidance prohibits deceptive subject lines, and misleading ones drive complaints.
- Match the style to the email type; a receipt, a newsletter and a sale need different approaches.
- Test with clicks or conversions as the deciding metric, because Apple's Mail Privacy Protection distorts opens.
The inbox "envelope": three parts that work together
Subscribers decide whether to open based on three things they see before clicking:
- Sender name. Usually the strongest factor. People open email from senders they recognise and trust. Use your brand name or a consistent "Name at Brand" format.
- Subject line. States the topic, benefit or reason to open now.
- Preview text. Mailchimp defines it as "the part of your email that appears next to the subject line in recipients’ inboxes". Use it to complete or expand the subject, not repeat it, and keep it short because clients truncate it.
Write all three together. A subject line like "Your March SEO report" works with the preview "Traffic is up, and two pages need attention" in a way neither does alone.
Seven principles for better subject lines
- Be specific. "5 ways to cut your heating bill this winter" beats "Winter tips".
- Front-load key words. On a phone, the end of a long subject line may never be seen.
- Write for one reader. Use the language your audience uses. Your buyer persona research helps here.
- Give a reason to open now when one genuinely exists: a deadline, limited stock, an event date.
- Personalise with care. Relevance beats inserting a first name. A subject based on what someone browsed or bought usually means more than a name token.
- Sound like your brand. Consistency with your brand voice builds recognition over time.
- Stay honest. The email must deliver what the subject promises.
Many of the same rules apply to headlines; see our guide to headline writing and our collection of copywriting formulas for structures you can adapt.
Subject line styles compared
Most subject lines fall into a handful of styles. None is always best; each suits different goals and audiences.
| Style | Example | Works well for | Watch out for |
|---|---|---|---|
| Direct benefit | Cut your ad spend waste in 15 minutes | Newsletters, B2B, how-to content | Overpromising |
| Specific topic | Your October website report | Reports, account updates, recurring emails | Can feel flat for promotions |
| Question | Is your website ready for AI search? | Educational content | Questions people can answer "no" to without opening |
| Curiosity | The mistake we made last quarter | Story-led newsletters with loyal readers | Turning into clickbait |
| Offer and deadline | Free delivery ends at midnight | Ecommerce promotions | False urgency |
| Personal or behavioural | New in the size you bought last time | Triggered and segmented emails | Feeling intrusive if data use is unexpected |
The legal line: no deceptive subject lines
In the US, CAN-SPAM covers all commercial email, not just bulk campaigns. One of the FTC's core requirements is plain:
“Don’t use deceptive subject lines.”
— Federal Trade Commission, CAN-SPAM Act: A Compliance Guide for Business
The FTC explains that the subject line must accurately reflect the content of the message, and penalties can be steep for each violating email. Fake "Re:" or "Fwd:" prefixes, invented order confirmations and false urgency are all risky. They also generate spam complaints, and Gmail and Yahoo expect bulk senders to stay below a 0.3% complaint rate. Our guide to email deliverability explains why complaints matter so much.
Subject line examples by email type
| Email type | Goal | Example subject line | Example preview text |
|---|---|---|---|
| Welcome | Confirm value, set expectations | Welcome! Here's your checklist | Plus what to expect from us each Friday |
| Newsletter | Highlight the main idea | The one Google Business Profile setting most owners miss | This week's tip takes five minutes |
| Product launch | Create interest | New: the lightweight trail shoe you asked for | Available in four colours from today |
| Sale | Clear offer and deadline | 20% off sitewide ends Sunday | Includes new arrivals |
| Abandoned cart | Gentle reminder | Still thinking about the oak desk? | We've saved your basket |
| Event invitation | Registration | Live Q&A: fixing slow WordPress sites, 12 March | Bring your questions; 45 minutes |
| Re-engagement | Confirm interest | Do you still want our emails? | One click keeps you on the list |
Triggered emails such as cart reminders and welcomes usually deserve the most testing, because they are sent to many people over time. See our guides to abandoned cart emails and email automation.
How to test subject lines, step by step
Open rates used to be the obvious test metric. Today they are distorted: Apple says Mail Privacy Protection "makes it harder for senders to learn about your Mail activity", and Mailchimp warns its open-rate benchmarks may be affected. Adjust your testing accordingly.
- Form a hypothesis. For example, "Naming the specific benefit will get more clicks than a curiosity-led line."
- Change one thing. Test specificity, length, a question versus a statement, or personalisation, not all at once.
- Choose the winning metric. Use click rate or conversions where your platform allows.
- Use a large enough sample. Small lists produce noisy results; consider running the same test across several sends.
- Send at the same time. Keep timing identical so only the subject line differs.
- Record the result. Keep a simple log of tests, results and sample sizes.
- Look for patterns. One win is a data point; repeated wins become guidelines for your brand.
Semrush's guide to A/B testing describes an estate-planning lawyer who found a personal subject line outperformed a formal checklist-style one with his audience. That is the point: what works depends on your readers, so test rather than copy. Our A/B testing guide covers sample sizes and significance in more depth.
Common mistakes
- Clickbait. It may lift opens once but erodes trust and raises complaints.
- Fake replies or forwards. "Re:" on a first-contact marketing email is deceptive.
- ALL CAPS and excessive punctuation. They look like spam to people, even if filters are more forgiving than they once were.
- Repeating the subject in the preview text. It wastes valuable space.
- Broken personalisation. "Hi {FirstName}" or a wrong name is worse than no personalisation; set fallbacks.
- Declaring winners on opens alone. Use clicks and conversions instead.
Related Guides
- Email Automation: Flows Every Business Should Set Up
- Marketing Automation: What to Automate and How to Start
- CRM for Marketing: How to Use Customer Data to Grow
Frequently Asked Questions
How long should an email subject line be?
There is no perfect length. Mobile inboxes show fewer characters than desktop, so put the most important words first and use preview text to add detail. Test lengths with your own audience.
Do emojis in subject lines help?
Sometimes. They can stand out in a crowded inbox but may look unprofessional to some audiences or render inconsistently. Use one where it adds meaning, and test it.
How do I A/B test subject lines if open rates are unreliable?
Use click rate or conversions as the winning metric where your platform allows, test with large enough samples, and repeat tests before treating a result as a rule.
Is it illegal to use a misleading subject line?
In the US, the FTC's CAN-SPAM guidance says subject lines must accurately reflect the content of the message. Misleading subject lines also drive spam complaints everywhere.
What is preview text?
The short line shown next to or under the subject line in many inboxes. Mailchimp describes it as the part of your email that appears next to the subject line; use it to add context rather than repeat the subject.
Conclusion
Great email subject lines are specific, honest and written for a particular reader. Treat sender name, subject and preview text as one message, match the style to the email type, and test with clicks or conversions rather than opens. Over time, your test log will tell you more about your audience than any list of formulas.
Want subject lines and emails that get opened and acted on? Our email marketing team writes, tests and optimises campaigns. Get a free quote.
