Subject Line

A subject line is the short line of text shown in the inbox that summarizes an email. Together with the sender name and preview text, it is most of what a recipient sees before deciding whether to open, so it has a direct effect on open rates.

What is a subject line in an email?

The subject line is a header field the sender writes when composing a message. Email clients display it in bold in the inbox list, next to the sender name. Its job is to tell the recipient what the email is about and give them a reason to open it now. Clear beats clever. A subject that says exactly what is inside is a safer bet than a vague teaser.

How long should a subject line be?

There is no fixed safe length. Each mail client and device truncates at a different point, and the available width changes how much text fits. Put the essential words first so the message survives truncation, keep it short, and check the draft in the clients your audience uses. Let the preview text carry the supporting detail.

Subject line examples

Welcome and onboarding. "Welcome to [Product], start here", "Send your first email in 5 minutes". More in our welcome email examples.

Product updates. "New: version history is here", "[Product] changelog: 3 things from June". More in our product update examples.

Receipts and billing. "Your [Product] receipt", "Payment received for your Pro plan". More in our payment confirmation examples.

Surveys and feedback. "Quick question about your first week", "2 minutes to make [Product] better?". More in our survey email examples.

How to write better subject lines

• Lead with the payoff: name the specific thing the reader gets by opening

• Match the body. A subject that overpromises teaches people to ignore you and invites spam complaints

• Use normal capitalization and at most one punctuation mark. All caps and stacked exclamation points look like spam to readers

• Personalize with meaning. A plan name or usage detail beats a first name pasted into a generic line

• Write 5 to 10 options and pick the clearest, then A/B test the top two

Subject line vs preview text

The subject line is the bold headline in the inbox. Preview text is the lighter snippet shown after it, pulled from the first text in the message unless you set it explicitly. They work as a pair: the subject earns attention, the preview text adds the context that tips the open. Writing them together avoids awkward inbox pairings like a sharp subject followed by "View this email in your browser". See preview text for examples.

Test your subject line

Paste a draft into our free subject line tester to check length, truncation, and common spam triggers before you send.

FAQ

What is a good subject line?
One that says specifically what the email contains and why it matters now, with the essential words first so it survives truncation. If the reader could guess the body from the subject, it is doing its job.

Do emojis in subject lines hurt open rates?
Not by themselves. One relevant emoji can add contrast in a crowded inbox. Several emojis, or emojis replacing words, read as promotional. Test before making them a habit.

Do capital letters trigger spam filters?
No single formatting choice decides that, and filters weigh many signals. ALL CAPS subjects look like shouting to readers, so sentence case with specific wording is the safer choice.

Should every email in a sequence have a different subject line?
Yes. Repeating the same subject makes follow-ups look like duplicates. Vary the angle while keeping the topic recognizable.

Related: Preview text, Open rate, Email header, A/B testing.