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A good email subject line says what is inside, puts the most important words first, and is short enough to read on a phone. Write the email first, sum it up in one plain sentence, cut that sentence to its first few words, and use the preview text to finish the thought. Then test one change at a time.
The test. If someone read only the subject line and the preview text, would they know what the email is about and why it matters to them? If yes, send it.
Why the subject line matters so much
In the inbox your email is three things: who it is from, the subject line and a few words of preview. People decide in a second or two. A subject line cannot rescue an email nobody wanted, but a weak one can bury an email people would have been glad to read.
It also affects what happens later. A subject that promises more than the email delivers gets opened and then marked as spam, and complaints hurt your sender reputation far more than a low open rate does.
Lead with the point
Most inboxes cut a subject line off, and on a phone only roughly the first 30 to 40 characters are shown. Whatever matters has to be in those first words.
| Instead of | Write | Why |
|---|---|---|
| Our October newsletter is here | 3 ways to cut your sending costs | Says what the reader gets |
| An important update about your account | Your invoice for October is ready | Names the actual thing |
| We are excited to announce a new feature | New: schedule a campaign for later | The news comes first |
| Don't miss out on this!!! | 20% off annual plans until Friday | A fact and a deadline, no shouting |
A method you can repeat
- Write the email first. You cannot summarise something that does not exist yet.
- Say what it is in one plain sentence, the way you would tell a colleague: "We have added scheduling, so you can set a campaign to go out on Tuesday morning."
- Cut it down to the part the reader cares about: "Schedule a campaign for Tuesday morning."
- Move the key words to the front and check the first 35 characters still make sense alone.
- Write the preview text as the second half of the sentence, not a repeat of the first.
- Read both together on a phone before you send.
Use the preview text
The preview text is the grey line after the subject. If you do not set it, the inbox fills it with whatever comes first in the email, which is often "View this email in your browser" or the alt text of your logo.
Treat it as a second subject line. If the subject is "Your October invoice is ready", the preview can be "It is $29, due on the 15th. Pay or download it here." Together they say everything a busy person needs.
What to avoid
- Tricks. "Re:" on an email that is not a reply, or "Your order" when there is no order. They work once, and then people stop trusting you.
- Shouting. All capitals and rows of exclamation marks look like spam to people and to filters.
- Vague teasers. "You won't believe this" asks for trust you have not earned.
- The same subject every time. "Monthly newsletter" tells nobody why this month's is worth opening.
- Personalisation with nothing behind it. A first name in the subject helps only when the rest of the email is also for that person.
Test one thing at a time
Opinions about subject lines are cheap, so test. Send two versions that differ in one way only (length, a number, a question against a statement) to two similar parts of your list, and compare.
Be careful with what you measure. Apple Mail and some other apps load images in the background, which counts as an open whether or not anyone read the email, so open rates are inflated and noisy. Clicks, replies and unsubscribes tell you more. And a test on a few hundred people rarely shows a real difference, so repeat it before you believe it.
None of this matters if the email never arrives. If your opens are unusually low across the board, check your SPF, DKIM and DMARC records and the state of your list before you rewrite another subject line.
Questions people ask
How long should an email subject line be?
Short enough that the point fits in roughly the first 30 to 40 characters, which is what most phones show. The line can be longer as long as nothing essential is in the part that gets cut off.
Do emojis in subject lines help?
Sometimes, for the right audience. One emoji that adds meaning can help a line stand out. Several, or one that replaces a word people need, makes the line harder to read. Test it on your own list.
Which words send an email to spam?
No single word does. Filters judge your sending reputation, your authentication and how people react to your mail far more than vocabulary. A misleading subject causes complaints, and complaints are what hurt.
Should I put the reader's name in the subject line?
Only if the email itself is personal. A name on top of a generic newsletter reads as a trick. Relevance matters more than a name.
What is preview text?
It is the short line shown after the subject in the inbox. You can set it yourself. If you do not, the inbox uses the first words of the email.
In Glai every campaign has its own subject and preview text fields, and the AI tab in the editor can draft an email from a short brief. Start free and send your first campaign today.