How to write subject lines that get opened

A method for email subject lines a Shopify store's subscribers open: one clear promise, preview text that finishes it, an honest from name, and a fair A/B test.

What this is for

The subject line, the preview text and the from name are the whole pitch for an email. They sit together in the inbox, and together they decide whether the rest of your work gets seen. This guide treats them as one unit, written after you know what the email is for.

There is one hard rule before any craft: in the US the subject line has to match what is inside. A line that tricks people into opening is not clever, it is a compliance problem. source, checked 28 September 2026

Time needed and what you need

Time needed: about 30 minutes per campaign. Ten minutes to draft five or six lines, ten to pick two and write preview text, ten to set up the test and check it on a phone.

The finished email, or at least its one main point and offer
Your email platform's campaign editor with access to A/B testing
A phone with your usual mail app, for a test send
A running note of past subject lines and how they did

The steps

1. Name the one reason to open

Write one sentence that says what the reader gets from this email: the new scent is back, the sale ends tonight, here is how to fix a wobbly table leg. If you cannot write that sentence, the email is doing too much, and no subject line will rescue it. Fix the email first.

2. Draft five or six lines, then cut

Write several versions fast: one plain, one with the product name, one as a question, one with a number such as a date or a size, one in your founder's voice. Then cut any that promise more than the email delivers. Put the important words first, because a phone shows far fewer characters than a desktop and the end of a long line simply disappears.

Plain: Linen sheets are back in sage
Specific: Sage linen, all four sizes, restocked today
Question: Still looking for the sage sheets?

3. Write preview text that finishes the thought

The preview text is the grey line after the subject. If you leave it blank, the inbox grabs whatever text comes first in the email, which is often an image description or the navigation links. Use it for the detail the subject line had no room for: the deadline, the price, the reason. source, checked 28 September 2026

Mind the numbers. Gmail can strip commas from figures in preview text, so write "over 10k reviews" rather than a long number with commas in it. source, checked 28 September 2026

4. Choose the from name on purpose

People decide on the sender before they read the subject. Your shop's name is the safe default; a person at the shop, such as "Maya at Fernhill", can feel warmer for letters and launches. Keep the subject out of the from name itself. Gmail's sender guidelines ask for the display name to identify the sender and nothing else. source, checked 28 September 2026

5. Drop the tricks

A fake "Re:" or "Fwd:" gets a click once and a spam report after. Gmail names it directly as something not to do. source, checked 28 September 2026

The same goes for all capitals, strings of exclamation marks and invented urgency. A deadline is fine when it is real. "Last chance" on a sale that runs again next week teaches people to ignore you.

6. Use an emoji only if it survives the test send

An emoji can make a line stand out, but it renders differently in each mail app and some show an empty box instead. Screen readers also read it aloud. Send yourself a test, look at it on your phone and on a desktop client, and drop it if it looks wrong anywhere. source, checked 28 September 2026

7. Test two lines that differ in one way

Pick your two best lines and run them as an A/B test in the campaign. Change only the subject: same preview text, same from name, same send time. If both lines differ in three ways, you learn nothing about which one mattered. source, checked 28 September 2026

Klaviyo suggests open rate as the winning metric for subject line tests. If your list is large enough, send the test to a slice of the audience and let the winner go to the rest. A score from the subject line tester is a quick check before the test, not a replacement for it. source, checked 28 September 2026

8. Read the result with Apple opens in mind

Apple Mail Privacy Protection loads emails automatically, which counts as an open whether or not a person looked. That inflates open rates and makes close tests harder to call. When the open rates of two lines are near each other, check clicks and orders before you declare a winner. source, checked 28 September 2026

9. Log every line and what it did

Keep a simple table: date, audience, subject, preview text, opens, clicks, orders, and which line won. After ten or so tests you will see your own patterns, such as product names beating puns or questions losing to plain statements, and those patterns are worth more than any general rule.

A worked example

For a skincare brand with 22,000 subscribers testing a restock email
Worked example (an invented brand, with invented numbers)
Line A reads "Our vitamin C serum is back". Line B reads "The serum that sold out in 3 days is back". Preview text, from name and send time are the same for both.
The test goes to 20% of the list each: 4,400 people get A and 4,400 get B. After six hours A has 1,012 opens and 142 clicks, and B has 1,188 opens and 171 clicks. Because Apple opens inflate both open counts, the brand checks clicks as well, and B wins on both.
The remaining 13,200 subscribers get line B (4,400 + 4,400 + 13,200 = 22,000). The brand logs it: specific proof beat a plain announcement for a restock.

Common mistakes

Writing the subject line first and the email second. The line then promises something the email does not quite deliver, and readers notice on the first click.

Testing two lines that also differ in send time or preview text. Any difference in results could come from either change. Klaviyo's advice is to test one factor at a time. source, checked 28 September 2026

Calling a test on opens alone when a large share of your opens come from Apple Mail. Klaviyo suggests extra care above a certain share and recommends a custom report that separates those opens. source, checked 28 September 2026

Reusing a line that won once in every campaign. A line that worked for a restock can fall flat for a launch, and the same words on every send stop being noticed.

Doing this with Tilly

Tilly's email marketing agent writes campaign emails, subject line and preview text included, for 10 credits a campaign. Each one goes to Klaviyo as a draft and nothing is sent without your approval, so you can still edit the line and set up the test in Klaviyo before it goes out.

Questions people ask

Does putting the subscriber's first name in the subject line help?

Sometimes, and it is easy to test. Klaviyo lists first-name personalisation as a subject line test to try, using a fallback such as "Friend" for profiles with no name. Run it as an A/B test against the same line without the name. source, checked 28 September 2026

Should the subject line be a question or a statement?

Neither wins everywhere, which is why it is one of the tests Klaviyo suggests starting with. Write both versions of the same idea and let your list decide. source, checked 28 September 2026

What if my open rates are low no matter what I write?

Then the subject line is probably not the problem. Look at who you send to and how often: an unengaged list or an inbox placement problem hides good lines. Start with cleaning your email list.

See this on your own store

Paste your store URL. The first pass takes about a minute and needs no account. Save a card (nothing is charged) and Tilly reads the whole store and works out who buys from you.