How to Write Email Subject Lines People Open

You can write a great email and still lose most of your readers before they see a single word of it. The subject line decides whether your message gets opened, skipped or deleted, and you get a few seconds of attention to make that call go your way.

The good news: strong subject lines aren’t about clever wordplay. They’re about being clear, specific and believable to the person scrolling their inbox. This guide gives you formulas that work, examples you can adapt, and the habits that land emails in spam or put you on the wrong side of the rules.

What makes a subject line work

Every subject line that gets opened does at least one of these things:

• Tells the reader what’s inside. “Your March invoice is ready” beats “Important update.”
• Gives a reason to open now. A real deadline, a limited event date or a timely answer.
• Feels written for them. It speaks to a problem or interest your audience actually has.
• Matches what they signed up for. Subscribers open emails that deliver what they expected when they joined.

Clarity usually beats cleverness. A subject line that makes readers guess often loses to one that simply tells them the benefit.

Keep it short and front-load the key words

Inboxes cut subject lines off, and phones show fewer characters than desktop screens. Mailchimp’s own guidance on catchy subject lines suggests keeping them under 40 characters so the full line displays. You don’t need to count every character, but you should:

• Put the most important words in the first few words.
• Cut filler like “Newsletter,” “Update” or your company name if it’s already in the sender name.
• Read it on your phone before sending.

Before: “The Smith Plumbing October Newsletter: Our Tips for Preparing Your Pipes for Winter” After: “Stop frozen pipes before the first freeze”

9 subject line formulas with examples

Use these as starting points. Swap in your own product, audience and voice.

A few tips for each:

• Numbers work best when they’re honest. “5 fixes” should contain exactly 5 fixes.
• Questions should be ones your reader would say yes to.
• Deadlines must be real. If “last chance” isn’t the last chance, readers learn to ignore you.
• Curiosity needs a hint of the payoff. Pure mystery (“You won’t believe this”) reads like spam.
• Personalization with a first name can help, but use it sparingly. A merge tag that fails and shows “Hi FNAME” looks careless.

Key takeaway: The best subject line is a promise you keep. Tell readers what they’ll get, say it in as few words as possible and make sure the email delivers exactly that.

Use preview text as your second subject line

Preview text (also called preheader text) is the short snippet many inboxes show beside or under the subject line. If you don’t set it, many email clients pull in whatever comes first in your email, which may be “View in browser” or an image caption.

Write it on purpose. Mailchimp’s help center describes preview text as a way to expand on your subject line, and suggests keeping it short with the most important detail first.

Good pairings:

• Subject: “Stop frozen pipes before the first freeze” Preview: “3 things to do this weekend, plus when to call us”
• Subject: “New: Saturday appointments are open” Preview: “Book online in under a minute. Slots fill fast.”

Don’t repeat the subject line in the preview. Use it to add the detail that tips readers into opening.

Spam-trigger habits to avoid

Modern spam filters weigh your sending reputation, authentication and how readers engage, not just individual words. But some habits signal “spam” to both filters and people. Avoid these:

• Fake “Re:” or “Fwd:” prefixes. Google’s email sender guidelines say not to start subject lines with Re: or Fwd: unless the message is an actual reply or forward.
• Misleading subject lines. The FTC’s CAN-SPAM guidance says a subject line must accurately reflect the content of the message. This applies to business-to-business email too. This is not legal advice.
• ALL CAPS and stacks of exclamation points. Mailchimp’s best practices advise against all capital letters, too many exclamation points and gimmicky words or phrases.
• Emoji pretending to be something else. Google’s guidelines say not to use emojis or non-standard characters to imitate graphic elements, or to place them next to a brand name to suggest verified status. One relevant emoji is fine; a row of them isn’t.
• Hype phrases. “Act now!!!”, “100% free”, “Guaranteed” and “You’ve been selected” look like scams because scammers use them. See our guide on spotting phishing emails for what readers are trained to distrust.
• Bait-and-switch. A subject promising a discount that doesn’t exist in the email drives spam complaints, and complaints hurt every future send.

Spam complaints matter. Google tells bulk senders to keep their user-reported spam rate below 0.10% and to avoid ever reaching 0.30%. Honest subject lines are one of the easiest ways to stay well clear of those numbers. If your emails are landing in spam despite good subject lines, check your email deliverability and SPF, DKIM and DMARC setup.

How to test subject lines

Guessing is fine for your first few sends. After that, test.

1. Pick one variable. Test subject lines only, not subject plus send time.
2. Write 2 versions with a real difference. For example, benefit-led versus question-led, not “Spring sale” versus “Spring Sale.”
3. Send each to a portion of your list. Most email platforms do this for you. Mailchimp, for example, lets you test up to 3 variations of a subject line and automatically send the winner to the rest of your list.
4. Choose the right winning metric. Opens are the obvious choice for subject lines, but they’re less reliable than they used to be.
5. Record what you learn. Keep a simple log of what won, so patterns emerge over time.

Why open rates aren’t the whole story

Apple’s Mail Privacy Protection downloads email content in the background by default, which means senders can’t reliably tell whether the person actually opened the message. For many lists, that inflates open rates. Look at clicks, replies and sales alongside opens, especially when results are close.

A quick checklist before you hit send

Where to start

If you’re rewriting subject lines for an existing list, start with your automated emails. They send every day, so improvements compound. Our guide to email automation workflows shows which flows to prioritize. Then:

1. Pull your last 10 campaigns and sort them by clicks, not just opens.
2. Look for patterns in the top 3: length, formula, topic.
3. Write your next subject line using the winning formula.
4. A/B test it against a different formula.
5. Repeat for a month and build your own playbook.

New to email entirely? Our first-month email marketing plan covers list building, platform setup and your first sends.

Conclusion

Strong email subject lines are clear, short and honest. Front-load the benefit, use preview text to add detail, test one formula against another and drop the tricks: no fake replies, no false urgency, no shouting. Readers reward emails that keep their promises, and so do the inbox providers that decide whether you reach them at all.

Sources

• Google Workspace Admin Help: Email sender guidelines — Re:/Fwd: rule, emoji and misleading header guidance, spam rate thresholds
• FTC: CAN-SPAM Act: A Compliance Guide for Business — subject lines must reflect message content
• Mailchimp: Best practices for Mailchimp emails — avoiding all caps, excess exclamation points and gimmicky phrases
• Mailchimp: About preview text — how preview text displays and how to write it
• Mailchimp: Create an A/B test — testing up to 3 subject line variations
• Apple: Mail Privacy Protection — background loading of remote content and its effect on open tracking