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 what a recipient sees before opening, which makes it the single biggest influence 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 almost always outperforms a vague teaser.

How long should a subject line be?

Aim for 30 to 50 characters. Mobile inboxes truncate around 30 to 40 characters, and desktop clients show roughly 60. Put the important words first so the message survives truncation, and 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 trains people to ignore you and drives spam complaints

• Use normal capitalization and at most one punctuation mark, all caps and stacked exclamation points are spam signals

• 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 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 length guidance and 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, in about 30 to 50 characters. If the reader could guess the body from the subject, it is doing its job.

Do emojis in subject lines hurt open rates?
Used sparingly, no. One relevant emoji can add contrast in a crowded inbox. Several emojis, or emojis replacing words, read as promotional and can hurt with both readers and spam filters.

Do capital letters trigger spam filters?
ALL CAPS subjects are a classic spam signal and feel like shouting. Normal sentence case with specific wording is safer and performs better.

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.