This guide is for the person who writes the subject line last, at 4:55pm, just before hitting send: the small-business owner sending a monthly newsletter, the marketing coordinator scheduling a product announcement, the nonprofit communications officer asking for donations. The subject line is the only part of your email most recipients will ever read. It decides whether the rest gets a chance. It is also the one line where cleverness most often backfires.
What a subject line has to do
In a crowded inbox, a reader scans three things in about the time it takes to scroll: who sent it, the subject line and, in many email apps, a line of preview text. Your subject line has two jobs:
1. Tell the reader what the email is about.
2. Give them a reason to open it now rather than later or never.
Everything else, from length to emoji, is secondary to those two jobs.
The legal floor: do not deceive
In the United States, the CAN-SPAM Act sets rules for commercial email. The Federal Trade Commission’s compliance guide for business lists “Don’t use deceptive subject lines” as one of its main requirements: the subject line must accurately reflect the content of the message.
The FTC guide makes several other points relevant to anyone writing marketing email:
- The law covers all commercial messages, not just bulk email, and makes no exception for business-to-business email.
- Each separate email in violation can bring penalties of up to $53,088, according to the guide’s inflation-adjusted figure.
- When an email mixes commercial and transactional content, the subject line matters: if a recipient reasonably reading it would conclude the email is an advertisement, it is treated as a commercial message.
Other countries have their own rules. Even where the law is looser, the practical rule is the same: a subject line that tricks someone into opening an email burns trust for every email after it.
Subject lines that cross the line
- “Re: your order” on a promotional email with no prior order
- “Your account has been suspended” to drive traffic to a sale
- “Final notice” when nothing is final
These are deceptive. They also train readers to distrust your name.
Five reliable subject line patterns
1. The plain description
Say exactly what is inside.
- October newsletter: three new recipes and a baking class date
- Your invoice for September is ready
Plain descriptions rarely look exciting on paper. In practice they work well for audiences who signed up because they want the content.
2. The specific benefit
Lead with what the reader gets.
- Cut your weekly meal-prep time with one freezer plan
- A checklist for your first tax return as a freelancer
3. The question the reader is already asking
- Is your website ready for the holiday rush?
- Which pension option suits a career break?
Use this only when the email genuinely answers the question.
4. The number
Numbers make the size of the content concrete.
- 5 mistakes we see in first-time grant applications
- 3 dates to put in your calendar this quarter
5. The real deadline
Urgency works when it is true.
- Workshop registration closes Friday at noon
- Last day for 20% off: ends midnight tonight
If the sale is extended next week, readers will notice, and your next “last day” will mean less.
Write the subject line you would be comfortable seeing next to the email’s actual content in a screenshot.
Comparing weak and strong versions
| Weak | Why it fails | Stronger |
|---|---|---|
| Newsletter #14 | Says nothing about the content | Garden newsletter: what to plant before the first frost |
| You won’t BELIEVE this!!! | Vague, shouty, spam-like | Our new loyalty scheme starts Monday |
| Quick question | Misleading if it is a sales pitch | Would a free website audit help your shop? |
| Important update | Overused, unspecific | Opening hours change from 1 November |
| Hi | No information | Your quote for the kitchen refit |
The businesses and offers above are examples for illustration.
Length, preview text and the sender name
Length
Many email apps cut long subject lines, especially on phones. The exact cut-off varies by app and screen size, so put the most important words first. “Opening hours change from 1 November” survives truncation; “We wanted to let you know about some changes to our opening hours” does not.
Preview text
Most email tools let you set the preview text, the line that appears after the subject in many inboxes. Use it to extend the subject rather than repeat it. If you leave it blank, many apps show the first line of the email, which may be “View this email in your browser”.
Sender name
The “From” name is read before the subject. CAN-SPAM requires header information, including the “From” name, to be accurate and to identify the sender. Use a name recipients recognise: the business name, or a person’s name plus the business.
When an email mixes a receipt and a promotion
Many businesses add a promotion to an order confirmation or account statement. The FTC guide explains how it decides the “primary purpose” of such an email. It gives two examples with the same subject line, “Your Account Statement”. In the first, the account details come first and a one-line promotion sits at the end; the FTC says that is most likely a transactional message. In the second, the promotion fills the top of the email and the order update sits at the bottom; the FTC says that is most likely commercial and subject to all of CAN-SPAM’s rules, despite the transactional-sounding subject.
The writing lesson is simple. If the email is a receipt, the subject line and the opening should be about the receipt. If the real goal is to sell, write a subject line that says so, include the required opt-out and postal address, and send it as a marketing email.
Personalisation and emoji
Personalisation
Inserting a first name can help, but test it: broken merge fields (“Hi FNAME”) look careless. Personalisation based on what the reader did, such as a product they viewed or a course they started, is usually more meaningful than a name.
Emoji
An emoji can add colour, but it displays differently across apps and can look out of place in professional contexts. If you use one, make sure the subject line still makes sense without it.
Testing subject lines
Most email platforms allow A/B testing: sending two subject lines to small portions of a list and the better performer to the rest. To get value from it:
1. Test one difference at a time, such as a question versus a statement.
2. Use a large enough sample; tiny lists produce noisy results.
3. Judge by clicks or replies as well as opens. A subject line that gets opens but no clicks may be overpromising.
4. Keep notes so lessons build up over time.
Open rates are also an imperfect measure, because some email apps load images in ways that can register an open the reader never made. That is another reason to weigh clicks and replies.
Subject line checklist
- Accurately reflects what is inside the email
- Most important words in the first few
- Specific: a topic, number, date or benefit
- Urgency only when the deadline is real
- Preview text set to extend, not repeat, the subject
- Sender name recognisable and accurate
- Merge fields tested; emoji optional, never essential
- No fake “Re:”, “Fwd:” or false account alerts
Takeaway
A good subject line tells readers what the email contains and why it is worth opening now. Be specific, put key words first, use real deadlines only, and never let the subject line promise something the email does not deliver. In the US, that last rule is also the law.
