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GuideUpdated

How to write subject lines people can act on

A good subject line makes a specific promise the email can keep. Write it after you know the offer, keep it short enough to scan, use preview text to continue the thought and test one change at a time. Never use a misleading subject to win an open; the message still has to be worth opening.

Key facts

  • Mailchimp's help center recommends no more than nine words and sixty characters for a subject line. Source
  • Mailchimp's help center recommends no more than three punctuation marks in a subject line. Source
  • The CAN-SPAM Act prohibits deceptive subject headings in commercial email. Source
  • Mailchimp's help center recommends using no more than one emoji at a time when emojis are appropriate. Source

Write and review an email subject line

  1. State the promise. Write down what the email actually offers: a new product, a restock, a useful guide, a cart reminder or a service update. If you cannot state the promise in one sentence, the subject line is not the problem yet.
  2. Write the specific version first. Start with the concrete detail: the product name, the benefit or the reason the message exists. A specific line is easier to shorten later than a clever line is to make specific.
  3. Cut it to a scannable length. Read the line on a phone-sized screen and remove words that do not change the meaning. Mailchimp's guidance is a useful starting point, not a law: aim for a line that can be understood at a glance.
  4. Write the preview text as a continuation. The preview text is the second line the inbox shows. Use it to continue the subject's thought or add the detail that did not fit, rather than repeating the same words.
  5. Check the claim and the punctuation. Make sure the email keeps the promise and that the subject is not misleading. Remove excessive punctuation and more than one emoji. The CAN-SPAM Act treats a deceptive subject heading as a violation, not a clever tactic.
  6. Test one change at a time. Compare two versions that differ in one meaningful way, such as the offer or the specificity, and keep the rest of the email the same. A test with several changes at once does not tell you which change mattered.

What this guide covers

This guide is part of the copywriting hub. It covers the subject line and the preview text as a pair. It does not promise a higher open rate, because the result depends on the list, the offer and the timing. It does describe the decisions that make a line worth opening.

The subject line is a promise

A subject line is a small contract. It tells the reader what is inside and asks for a moment of attention. If the email does not keep the promise, the reader learns to distrust the next one. That is why the best subject lines are specific rather than clever: specificity is easy to keep.

Start with the reason the email exists. A restock, a new product, a cart reminder and a shipping update are different messages. A subject line that could introduce any of them is not specific enough. The first draft should name the thing, even if the final line is shorter.

The promise also sets the tone. A playful line on a serious service update is a mismatch. A formal line on a birthday offer is a missed opportunity. The subject line should sound like the brand and fit the message, not like a separate piece of copy glued to the top.

Specific beats clever

Clever lines are memorable when they work and confusing when they do not. A specific line gives the reader a reason to open without requiring them to decode a joke.

“The linen shirt is back in three colours” is specific. “You'll want to see this” is not. The first line tells the reader whether the email is for them. The second asks them to spend attention to find out. On a crowded phone screen, the first line wins more often.

Specificity also helps the approval review. An approver can check whether the email delivers the shirt, the colours and the restock. A vague line is harder to review because there is nothing concrete to verify.

Length is about the cut, not the count

Mailchimp's help center recommends no more than nine words and sixty characters. That is a useful starting point, but the real question is what the inbox cuts off. The sender name, the subject line and the preview text compete for the same small space.

Write the line, then read it as if the last few words were missing. If the meaning survives, the length is probably fine. If the important word is at the end, move it forward. The same test works for the preview text.

A short line is not automatically better. “Sale” is short and says almost nothing. “The summer sale ends Sunday” is longer and gives the reader a reason to act. The goal is not the fewest characters; it is the most meaning that survives the cut.

Preview text is the second half

The preview text is not a place to repeat the subject line. It is the continuation. If the subject names the product, the preview can name the benefit or the deadline. If the subject asks a question, the preview can hint at the answer.

A good pair reads like one sentence split across two lines. A bad pair reads like two unrelated headlines. The inbox shows them together, so write them together.

The preview text also has a length limit in most clients. Keep the important words early. A preview that only makes sense after the cut is not doing its job.

Punctuation, emojis and other signals

Punctuation is a signal, and too much of it looks like shouting. Mailchimp's guidance is to use no more than three punctuation marks per subject line, and no more than one emoji at a time. Those are reasonable defaults, not a guarantee of good taste.

An emoji can add tone or make a line easier to scan. It can also render differently across clients, look unprofessional in a serious message or push the line over the cut. Use one only when it adds meaning the words do not already carry.

Capitalisation matters too. A line written in all caps reads as urgent or angry. Excessive exclamation marks have the same effect. The safest approach is to write the line the way you would say it to a customer, then remove one exclamation mark.

Honesty is not optional

The CAN-SPAM Act prohibits deceptive subject headings in commercial email. That is a legal rule, not a style preference. A subject line that promises a free gift the email does not offer, or a deadline that does not exist, is a violation and a trust problem.

Misleading lines can also hurt deliverability. A list that reports a message as spam teaches the receiving server that the sender is unreliable. The short-term open may cost more than it earns.

The approval step is the place to catch a misleading line. An approver who can see the email and the subject together can ask whether the promise is kept. A subject line approved in isolation is easier to get wrong.

Test the line, not the myth

A subject-line test compares two versions of the same email with one meaningful difference. If one version names the product and the other uses a generic benefit, the test tells you which approach the list responds to. If one version also changes the offer, the result is not a subject-line result.

Use clicks and revenue as the outcome, not opens alone. An open can be inflated by a privacy feature or a scanner. A click is closer to the decision you care about. The revenue per email page explains the attribution model.

Keep a record of what you tested and what happened. A list's response changes over time, and a line that worked last season may not work now. The record is more useful than a folder of “winning subject lines” copied from someone else's account.

Review with the whole message

The final check is the whole message: sender name, subject, preview, first screen of the email and the offer. A subject line cannot rescue an email that does not deliver what it promises. It can only bring the right reader to the message.

The abandoned checkout guide applies these ideas to a flow where the subject has to name the cart. The approvals page explains how the review step fits the send path.

Frequently asked questions

How long should a subject line be?

Mailchimp's help center recommends no more than nine words and sixty characters. That is a useful default, not a universal rule: a transactional subject may need a reference number and a short promotional line may work at a different length. The practical test is whether the meaning survives on a phone screen. If the important words are cut off, shorten the line before you send it.

Do emojis improve open rates?

There is no single answer that applies to every list. Mailchimp's guidance is to use no more than one emoji at a time, and to use it carefully because too many special characters can make a message look like spam. Test an emoji against the same line without one, and judge the result on clicks and revenue rather than opens alone.

Can I use urgency in a subject line?

Only when the urgency is true. A sale that ends tonight can say so; a permanent discount dressed up as a final warning is deceptive and can violate the CAN-SPAM Act's prohibition on misleading subject headings. Urgency also wears out. A list that sees a countdown every week learns to ignore it, and the next real deadline gets less attention.

Should I put the customer's name in the subject line?

It can work, and it can also feel like a mail merge when the rest of the line is generic. Personalisation is strongest when it uses information the customer recognizes as relevant, such as a product they viewed or a cart they left. Test a personalised line against a specific non-personalised line rather than against a vague one.

What should the preview text say?

The preview text should continue the subject line, not repeat it. If the subject names the product, the preview can add the benefit or the reason to act. Keep it short enough that the inbox does not cut it off, and treat it as part of the same promise. A subject line that says one thing and a preview that says another is a bait and switch.

Sources