Email Subject Lines: How to Write Them, Ideal Length, Capitalization and Examples

The subject line is the part of an email that nearly everyone reads and many people read first. It decides whether your message is opened now, later or never, and it also affects whether filters trust you. People search for "email subject lines" in great numbers, usually hoping for a list of clever phrases. Clever phrases rarely matter as much as three basics: clarity, honesty and fit with the sender and the audience. This guide explains how to write a good email subject line, how long it should be, how to capitalise it, whether to use emoji, numbers or personalisation, what to avoid, how to test subject lines and gives examples for common situations.

What does a subject line do?

A subject line has three jobs. It tells the reader what the message is about, it helps them decide how urgently to open it and it helps them find it later. In bulk email it works together with the sender name and the preview text, the short snippet shown after the subject in many inboxes. Readers judge the three as a group, so write them together.

How to write a good email subject line

1. Start with the point. Put the most important words first. Mobile apps and narrow windows cut subject lines off, so the end may never be seen.
2. Be specific. "Your invoice for October" beats "Important update". "Agenda for Thursday's planning meeting" beats "Meeting".
3. Say what the reader gets or must do. Benefit or action: "Confirm your appointment by Friday", "A checklist for moving to a new mail provider".
4. Use the reader's language. Write the way your audience speaks, not the way your internal team talks.
5. Match the content. If the message does not deliver what the subject promises, readers feel misled and complaints rise.
6. Fit the sender. A message from a friend, a bank and a newsletter each needs a different voice.
7. Read it on a phone. Check how it looks cut off.

How long should an email subject line be?

There is no exact limit that works everywhere. Different apps show different numbers of characters, and phones show less than desktops. A common rule of thumb is to keep the subject to roughly forty to sixty characters and put the key words at the front, so that the meaning survives a cut. Longer subjects are not forbidden, since email standards allow much more, but anything that matters should come early. Treat the figure as a guide and test it with your own readers.

Should email subject lines be capitalised?

Both styles are acceptable. Choose one and use it consistently.

For business email, sentence case is a safe choice. Capitalise proper nouns and the first word. Avoid capitalising whole words for emphasis.

Emoji, numbers, questions and personalisation

• Emoji. Some brands use one to add tone. They may display differently across devices and can look unprofessional in formal contexts. Use sparingly and test, and never let an emoji replace meaning.
• Numbers. "5 ways to improve your deliverability" sets a clear expectation, as long as the message really contains five.
• Questions. A relevant question can invite attention, while a vague or misleading question feels like bait.
• Personalisation. A first name or a reference to something the reader did can help, if the data is correct. A wrong or empty name is worse than none, so make sure fallbacks work.
• Urgency. Use it only when it is real. "Ends tonight" for a sale that is actually ending is fine. False urgency teaches readers to ignore you.

What to avoid in a subject line

• Deception. Fake "Re:" or "Fwd:" prefixes, claims that a message is a reply when it is not and promises the email does not keep are misleading. In the United States, the CAN-SPAM Act prohibits deceptive subject lines in commercial email, and other countries have similar rules. This is general information, not legal advice.
• Shouting and symbol clutter. Capital letters, many exclamation marks, dollar signs and repeated punctuation look like spam.
• Vague words. "Hello", "Update", "Quick question" and "Important" tell the reader nothing.
• Overpromising. "You won't believe this" invites disappointment.
• Very long subjects with the point at the end.
• Misleading personalisation, such as faking familiarity.

Do spam trigger words matter?

Lists of forbidden words circulate widely, but modern filters weigh many signals together: your authentication, your reputation, engagement, the links, the overall content and whether recipients complain. A single word seldom decides the outcome. Still, subject lines full of hype, money claims and capital letters look like the messages filters were built to catch. Write honestly and check your whole message. Read how content affects spam filters and how to test if your email will go to spam.

Email subject line examples by situation

Business and work

• "Agenda for Thursday's planning meeting"
• "Request: approval for the Q4 training budget"
• "Follow-up: notes from today's call"
• "Invoice 1043 for October"

Thank you

• "Thank you for your time today"
• "Thanks for the introduction to Maria"

Follow-up and reminders

• "Reminder: documents due Friday"
• "Following up on our proposal"
• "Did the new schedule work for you?"

Newsletters and announcements

• "This week: three changes to our API"
• "October update: new reports in your dashboard"

Welcome and onboarding

• "Welcome to Example: your first three steps"
• "Your account is ready"

For more on welcome emails, see welcome email examples.

E-commerce

• "You left something in your basket"
• "Your order has shipped"
• "Back in stock: the item you asked about"

Job applications

• "Application for Marketing Coordinator: Priya Sharma"
• "Following up: Marketing Coordinator interview"

For sales and cold outreach, see business, sales and cold email subject lines.

How to test subject lines

1. Choose one variable. Change only the subject line and keep the content, sender and send time the same.
2. Split the list randomly. Send each version to an equal share of a segment.
3. Judge by clicks and conversions, not opens alone. Mail privacy features inflate opens; see email open rate.
4. Send the winner to the rest, and record what you learned.
5. Repeat. One test can be a fluke. Look for patterns across several sends.

Subject line, sender name and preview text work together

Readers never see a subject line alone. In most inboxes it appears beside the sender name and a snippet of the first lines of the message. If the subject says one thing and the preview text repeats it, you waste valuable space. Use the subject for the main point and the preview text for the next detail. Set the preview text deliberately, since otherwise the app shows whatever comes first in the message, which is often a link to view the email in a browser. A recognisable sender name matters as much as the words: people open mail from names they trust.

Subject line checklist

• Does it say what is inside?
• Are the key words first?
• Is it honest?
• Does it read well on a phone?
• Is the capitalisation consistent?
• Does it work with the sender name and preview text?
• Would I open this if I received it?

References and further reading

• FTC: CAN-SPAM Act compliance guide for business
• RFC 5322: Internet Message Format
• RFC 2047: MIME Part Three, Message Header Extensions