How do you write marketing email subject lines that get opened?

Give the reader a specific, truthful reason to open the email. Lead with the useful product, benefit or news; preserve offer conditions; and check how the line appears beside the sender and preview text. Treat length, names and emojis as choices to test. No wording guarantees an open, and recorded opens alone do not establish human interest or sales.

Start with the email you are sending. What will the recipient find inside that makes it worth reading? Write that answer before reaching for a formula.

This guide covers marketing emails to your subscribers and customers. If your question is whether a generator writes better candidates, see the AI-versus-human subject-line evidence. Here, the job is to make the candidate accurate, useful and ready to compare.

Start with the promise the email can keep

Nielsen Norman Group’s subject-line guide recommends telling recipients what is inside so they can make an informed decision. That gives you a practical starting point: describe the reason to read, rather than announce that you have sent another newsletter.

Before drafting, write down:

• Who the message is for: the recipient group you actually selected.
• What they can get or learn: the product, offer, answer or update inside.
• What qualifies the promise: eligible products, availability, code and any real deadline.

Consider an invented offer: selected refill kits are 15% off with code REFILL until Thursday at 8pm Eastern. The email body and landing page carry those same terms.

A usable subject is “15% off selected refill kits.” It states the benefit and preserves the product restriction. The preview could add “Use REFILL before Thursday, 8pm Eastern.” Keep the complete terms in the email and destination too; a preview is not assured space for explaining an otherwise misleading subject.

“15% off everything” fails this brief because the offer is selective. “Your personal offer expires tonight” also fails unless the store actually has that personal deadline. Make the line more interesting within the facts you have.

Rewrite the vague hook into something the reader can judge

Use these as illustrative rewrites, not tested winners. Each follows its stated email content; there is no claimed performance improvement.

For each candidate, ask: could someone understand the subject without opening, and would the email fulfil that expectation? An interesting question can work as a draft, but its answer needs to be in the email. Curiosity does not justify inventing a benefit.

Keep alternatives tied to the same content. If one subject promises a discount and another promises a care guide, you need an email that honestly delivers both before comparing those lines on the same send.

How long should an email subject line be?

Use length as an editing check, not a performance promise. Mailchimp recommends no more than 9 words and 60 characters. Nielsen Norman Group’s 2014 guide recommends no more than 40 characters and putting meaningful words first, while noting that inbox display limits vary.

Those recommendations are different. Neither number guarantees that the full line will display on every device or that it will beat a longer line.

In the invented offer, compare these drafts:

• Buried meaning: “We’ve put together something special: 15% off selected refill kits.”
• Meaning first: “15% off selected refill kits.”

The second removes the introduction while keeping the benefit and restriction. That is a useful edit even before a performance test. Cutting “selected” merely to meet a character target would change the offer.

Read the first few words on their own. Then inspect a delivered test in the inboxes your team can check. Record where the text is cut off rather than treating a character counter as a rendering test.

Should you add a name or an emoji?

Use either only when it adds something useful. Mailchimp suggests trying name or location personalization; Nielsen Norman Group’s 2014 guide cautions that names can feel mass-produced and displace useful content. For emojis, its 2020 guidance favors context or emotional meaning, while Mailchimp advises at most one that supplements words. These are recommendations to test, not guaranteed gains.

If you use a name, review recipients with a missing or malformed value and approve a version that works without it. Compare that line with a descriptive alternative. “Sam, something special for you” still says less about the invented offer than “15% off selected refill kits.”

Check an emoji’s appearance across operating systems, as the same subject-line guidance advises. Remove the symbol during review: if the line no longer makes sense, revise the words.

Check the inbox version, then compare candidates

Read the sender, subject and preview together in a delivered test. Litmus explains that preview text appears beside or beneath the subject, with its display depending on the client and recipient settings; it recommends complementary copy rather than repetition. Some settings show no preview, so keep the main promise in the subject.

For the refill offer, check that “selected” remains clear, the deadline matches the scheduled send, and the preview adds the code and timing. Inspect the received version for broken names, unexpected text and emoji rendering before approving it.

Then compare a meaningful alternative on the same campaign. Keep the sender, preview, body, offer and sending conditions aligned and randomly split the eligible test audience. If you change the preview too, describe the experiment as testing the pair rather than the subject alone.

For a manual campaign test, Klaviyo documents Campaigns → Create campaign → Single channel → Email → Create A/B test above the subject field. Its guide says to change one factor at a time. Click rate is an available winning metric; placed-order rate requires a Placed Order metric and is unavailable with personalized variations.

Choose the outcome and common observation window before sending, and compare equivalent denominators. Apple’s privacy documentation explains why background content downloads can occur without engagement: a recorded open is not necessarily a read. Use the open-rate guide for those reporting limits and the subject-line test guide for a fuller comparison protocol.

Keep a short record beside the campaign:

This preserves what you approved and what the comparison actually tested. A click result answers an engagement question; it does not, by itself, establish a sales or retention gain.

Can open rate tell you which subject line is better?

It can show which version had more recorded opens, but those are not necessarily human reads. Apple’s Protect Mail Activity can download email content without engagement. Choose the click or purchase outcome relevant to your campaign and inspect opens as supporting information; more recorded opens alone do not prove more sales.