How to write email subject lines that get opened — preview text, personalisation and A/B testing
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Key takeaways
- Before opening a message, recipients see only three things: the sender name, the subject line and the preview text. Design all three together.
- A good subject line says clearly what is inside and why it matters, puts the key words first and matches the content; personalisation needs a fallback for empty name fields.
- All caps, strings of exclamation marks, exaggerated urgency and fake “RE:” or “FW:” prefixes erode trust; the US CAN-SPAM Act and Hong Kong’s UEMO both rule out misleading subject lines.
- Without preview text set, mail clients show the first text in the body, which is often something useless like “View in browser”.
- Apple’s Mail Privacy Protection stops senders seeing whether a message was opened, so judge subject-line A/B tests on clicks and conversions, not on open rate alone.
However good your newsletter is, it achieves nothing if nobody opens it — and recipients usually decide in a second or two, from a single line in their inbox. This guide covers how to write email subject lines: the three elements to design together, the principles of a good subject line, the common mistakes, examples by type of email, and how to A/B test your way to what works for your readers.
What recipients see before they open
In the inbox list, recipients usually see only:
- Sender name — who is this from?
- Subject line — what is it about?
- Preview text — a short line of extra context after the subject.
They are read together, so design them together. A great subject line from an unfamiliar sender may still be deleted unread; preview text that repeats the subject wastes the space. For the sender name, see the “Sender name and sending address” section of How to keep your email and newsletters out of the spam folder — a complete guide for business senders.
Some mailboxes also show a brand logo next to the sender, through the BIMI standard. For Gmail, the sending domain’s DMARC policy must be set to quarantine or reject (with pct at 100), and you need a VMC or CMC certificate from a third-party certificate authority (Google Workspace: Set up BIMI). It is optional, but it helps recipients recognise you.
Six principles of a good subject line
1. Say clearly what is inside and why it matters
Recipients need to see at a glance what opening the message will do for them. Rather than “September newsletter”, name the single most useful thing in this issue.
2. Put the key words first
On a phone, longer subject lines are often cut off after the first half. How much shows depends on the mail client, the device and even screen orientation, so there is no universal character limit. The rule is to put the most important words first — then check it on an actual phone.
3. Match the content
Whatever the subject line promises, the message has to deliver. An exaggerated subject line buys one open at the cost of the recipient’s trust in every message after it, and invites “Report spam” clicks.
4. Personalise with care, and plan a fallback
A recipient’s name or city in the subject line can make a message feel relevant. Two things to watch:
- When the name field is empty, the subject can turn into “Hi ,” or even expose a raw variable such as
$firstname$. Always set fallback text, such as “Hi there”. - Personalisation is more than a name. Tailoring content to past purchases, browsing or region often matters more than adding a first name.
5. Make the brand easy to recognise
If the sender name is already your brand, the subject line need not repeat it. If the sender name is a person, a brand name in square brackets, such as “[Brand]”, helps recipients place you.
6. Use one or two symbols or emoji at most
A symbol can catch the eye in a crowded inbox, but rendering varies across devices, and older mail clients may show a blank box. Test on several devices before sending.
Five common mistakes
1. All caps or heavy punctuation
”!!!”, ”???” or a whole sentence in capitals reads like shouting — and is a common pattern in spam.
2. Exaggerated urgency
“Last chance”, “act now” or “urgent” is fine for a real deadline now and then. Used on every message, recipients stop noticing, or start resenting it.
3. Misleading subject lines
Adding “RE:” or “FW:” to something that is not a reply or forward, or writing a subject line unrelated to the content, is misleading. It damages trust and can break the law: the US CAN-SPAM Act requires the subject line to accurately reflect the content of the message (FTC CAN-SPAM compliance guide), and Hong Kong’s Unsolicited Electronic Messages Ordinance also rules out misleading subject headings — see Hong Kong promotional messaging compliance checklist — what the UEMO requires of senders.
4. Spelling and grammar errors
The subject line is the most visible line in the whole message. A typo there makes people doubt the rest. Have a colleague read it before you send.
5. Too long, or too vague
“Latest news” or “Important update” could sit on any email, which is exactly why nobody opens it. Rather than counting characters, make sure the first half already says what matters.
Preview text (preheader)
Preview text is the short line shown after the subject in the inbox list. Most email marketing platforms give it a field of its own.
What happens if you leave it empty? The mail client takes the first text in the body — often “View in browser”, “Having trouble viewing this email?” or “You are receiving this because…” — and wastes the most valuable space after the subject line.
When writing preview text:
- Add to the subject; do not repeat it. If the subject is “Holiday gift boxes are open for pre-order”, the preview text can say “Order by 10 September for the early-bird price and get free delivery”.
- Put your second key point here — a deadline, who it is for, or a concrete figure (which must match the content).
- Like the subject line, preview text can be cut off on a phone, so lead with what matters.
Subject line examples by email type
These are illustrations; what works for your list is something only testing can show.
| Email type | Subject line | Preview text |
|---|---|---|
| Newsletter | Three ways to keep your newsletter out of the spam folder | This issue: sender names, preview text and unsubscribe design |
| Promotion | Holiday gift boxes — early-bird price ends 10 September | Free shipping over a minimum spend |
| Event invitation | 15 October webinar: email deliverability in practice | 45 minutes plus Q&A, limited places |
| Reminder | You still have 2 items in your cart | Stock is limited — check out in one click |
| Transactional | Your order #10234 has shipped | Expected in 2 business days — track it here |
Keep transactional subject lines plain and clear, with no promotional wording, so recipients can find them in their inbox at a glance.
How to A/B test subject lines
An A/B test sends two versions to two small, randomly split parts of your list, then sends the better performer to everyone else. The essentials:
- Change one variable at a time. If only the subject line changes — same sender name, preview text and send time — you know what caused the difference.
- Make the test groups big enough. With a small list, a handful of people can flip the result. Rather than settling it in one send, build up evidence across several.
- Decide the winning metric before you start.
Open rate can no longer be trusted on its own. Apple explains that with Mail Privacy Protection turned on, senders cannot see whether you have opened their message — though the protection does not extend to links (Apple Support: Use Mail Privacy Protection on iPhone). In other words, open data from Apple Mail users is unreliable, while click data is far more dependable. Compare subject lines mainly on click rate and downstream conversions, and treat open rate as a secondary signal.
The UFOSEND EDM dashboard can generate subject lines, preview text and body drafts from your campaign topic, audience and tone, which makes it quick to prepare the variants for an A/B test.
Subject lines and spam filtering
Subject lines do affect spam filtering, but they are not the main factor. Your sending domain’s authentication and reputation matter far more than any word in the subject line. The full picture is in How to keep your email and newsletters out of the spam folder — a complete guide for business senders.
FAQ
How long should an email subject line be?
There is no universal character count, because display length varies by mail client, device and screen orientation. Instead of counting, put the key words first and check on a phone what is left once it is cut off.
Can I use emoji in subject lines?
Yes, but one or two is plenty, and test how they render on different devices first. An emoji should add to the words, not replace them.
Should the subject line include my brand name?
If your sender name is already the brand, usually not. If the sender name is a person, adding the brand helps recipients recognise you.
Is a personalised subject line always better?
Not necessarily. Whether a first name helps is something to test on your own list. What matters more is a fallback for empty fields, and content that is genuinely relevant to the recipient.
Is it a problem to write “limited time” or “last chance” every time?
Yes. Recipients soon tune it out, so a real deadline loses its force — and if the deadline is not real, it is misleading.
Want to prepare subject lines and preview text faster? UFOSEND’s EDM email marketing service generates subject lines, preview text and body drafts you can refine with merge fields and links, and keeps your mail in the inbox with SPF/DKIM/DMARC alignment and Postmaster monitoring.