What is an email subject line?

Email subject line

An email subject line is the single line of text recipients see in their inbox before opening your message. It appears next to your sender name and determines whether someone opens, ignores, or deletes your email.

Think of it as your email's headline. You have roughly 40-60 characters to convince someone your message is worth their time. That's not much space, which is why every word matters.

Why subject lines determine your email's success

Your subject line is the gatekeeper. No matter how valuable your email content is, none of it matters if the message never gets opened.

Subject lines shape first impressions of your brand. When someone scans their inbox, your subject line competes against dozens of other messages. A weak one gets lost. A strong one earns attention and builds recognition over time.

Beyond opens, subject lines affect deliverability. When recipients consistently ignore or delete your emails, inbox providers notice. They may start routing your messages to spam. Engaging subject lines keep your sender reputation healthy.

What makes a subject line work

Effective subject lines share a few characteristics: they're clear about what's inside, they create enough curiosity or promise enough value to earn a click, and they match the content that follows.

• Clarity over cleverness: Recipients should understand what they'll get. "Your order shipped" beats "Something exciting is on its way!"
• Specificity: "5 ways to reduce cart abandonment" outperforms "Tips for your store"
• Relevance: A subject line about winter coats won't resonate in July, no matter how well-written

The best subject lines also feel personal. Using the recipient's name or referencing their behavior (like a recent purchase) signals that this email was meant for them.

Common subject line mistakes

Being vague: "Quick update" or "Checking in" tells recipients nothing. They'll skip it.

Overpromising: If your subject line promises a secret and your email delivers a product pitch, trust erodes fast. Misaligned subject lines drive unsubscribes.

Triggering spam filters: ALL CAPS, excessive punctuation (!!!), and words like "FREE CASH NOW" can land your email in spam. Write like a human, not a late-night infomercial.

Ignoring mobile: Most emails get opened on phones, where subject lines get truncated. Front-load the important information so it's visible even when cut off.

How to write better subject lines

Start with the benefit. What does the reader gain by opening? Lead with that.

A subject line like "Your weekly marketing report" is functional, but "Your open rates jumped 23% this week" gives a reason to click.

Questions can work well when they're specific. "Ready to automate your welcome sequence?" invites curiosity. "Want to learn something?" does not.

Test different approaches. A/B testing your subject lines reveals what resonates with your audience. Try varying length, tone, personalization, and urgency to see what moves the needle.

Types of subject lines that perform

Urgency-based: "Last chance: Sale ends tonight" works when the deadline is real. Overuse it and readers stop believing you.

Curiosity-driven: "The one metric most marketers ignore" prompts opens because readers want to know if they're making that mistake.

Benefit-focused: "Cut your email production time in half" promises a clear outcome.

Personalized: "Sarah, your cart is waiting" uses the recipient's name and references specific behavior.

Question-based: "Struggling with low open rates?" speaks directly to a pain point.

Mix these approaches based on your email's purpose. A re-engagement campaign might lean on curiosity, while a flash sale email needs urgency.

Subject lines for different email types

Welcome emails: Keep it warm and clear. "Welcome to [Brand]—here's what's next" sets expectations.

Promotional emails: Lead with the offer. "30% off ends Friday" is direct and actionable.

Newsletters: Tease the best content. "This week: The automation mistake costing you sales" gives readers a reason to open.

Transactional emails: Prioritize clarity. "Your order #4521 has shipped" tells recipients exactly what they need to know.

Win-back emails: Acknowledge the gap. "We miss you—here's 20% off your next order" combines emotion with incentive.

Testing and improving your subject lines

Don't guess. Test.

Run A/B tests on a portion of your list before sending to everyone. Compare open rates between two subject line variations, then send the winner to the rest.

Track patterns over time. You might discover that your audience responds better to questions than statements, or that shorter subject lines consistently outperform longer ones.

ActiveCampaign's subject line generator can help you brainstorm options when you're stuck. Use it as a starting point, then refine based on what you know about your audience.

FAQs

How long should an email subject line be?
Aim for 40-60 characters. Mobile devices cut off longer lines, so put the most important words first. For more detail, see our guide on subject line length.

Should I use emojis in subject lines?
They can boost visibility when used sparingly and appropriately for your brand. Test them with your audience before committing.

Do personalized subject lines really perform better?
Yes, when the personalization is relevant. Using someone's name or referencing their behavior signals that the email was crafted for them, not blasted to a list.

What words should I avoid in subject lines?
Skip spam triggers like "FREE," "Act now," or excessive punctuation. Also avoid vague phrases that don't communicate value.

Ready to see which subject lines resonate with your audience? Start your free ActiveCampaign trial and test your way to higher open rates.