Cold Email Subject Line Do's and Don'ts (Plus, 36 Examples)

“Don’t judge an email by its subject line”—said no one ever.

When it comes to cold outreach, subject lines can make or break your campaigns. That’s because they’re the most prominent thing people see in their inboxes. 

It’s a split-second decision: all it takes is your cold email subject line to decide whether your message is actually worth reading.

Because of this, a big part of your cold email campaigns’ success—open rates, clicks, and response rates—depends on your subject line. Without an enticing subject line, your cold outreach won’t get very far.

Below, we’ll cover the do’s and don’ts of writing good subject lines, and explain the reasoning behind each. We’ll also provide real cold email subject line examples that you can use or pull from for inspiration in your own cold email campaign.

Here's what we'll get into:

1. Do: Write short subject lines
2. Do: Personalize it
3. Do: Establish some kind of connection
4. Do: Pique curiosity about something relevant
5. Do: Make it self-evident
6. Do: Use power words to grab prospects’ attention
7. Do: Experiment
8. Don’t: Use false prefixes or clickbait
9. Don’t: Write vague subject lines
10. Don’t: Ask for something without offering anything in return
11. Don’t: Use all caps or a lot of exclamation marks
12. Don’t: Write very formally or use overly promotional wording

What makes an effective cold email subject line?

The best cold email subject lines capture a prospect’s attention by triggering one of these common motivations: 

• Self-interest: It offers something of value that the recipient feels they can benefit from.
• Emotional interest: It sparks a positive emotion—for instance, by making prospects curious or feel special.
• Relational interest: It fosters some kind of connection between you and the recipient, making them more interested in what you have to say. 

To write a good subject line, consider leaning into one of these elements. You can go about this in a number of ways—we’ll provide specific tactics and examples below.

Just one important note before we dive in: 

When you measure your cold emails’ performance, open rate isn’t the only metric that matters.

Consider the bigger picture, like what you’re asking prospects to do. The completion rate of this desired behavior, whether that’s clicking on a link in the body of your email, scheduling a call, or something else, ultimately matters more than the number of people opening your message. 

Why? Vague or clickbait subject lines often perform well in terms of open rate but fail in terms of conversion rate. So a higher open rate doesn’t mean much if conversions are nonexistent. It’s much more meaningful to get lower open rates but strong conversions.

How to write better cold email subject lines

At Demand Curve, we see a lot of cold emails because we receive them and because we teach marketers how to write them as part of our Growth Program. We’ve observed common patterns used by both good and bad emails, and we’ve tested them—so we know which ones actually drive impressive email marketing results. 

There are several approaches to writing an effective subject line—and what works for you will ultimately depend on who your audience is and what you’re offering. Below are seven tried-and-true tactics for writing a great subject line, plus five don’ts you should avoid.

For help creating the rest of your cold outreach, check out our post on writing cold emails.

Do: Write short subject lines

Reading emails on your phone is the new norm. In fact, according to a meta-analysis of several studies, an average of 63% of people regularly open their emails on mobile devices.

What this means for your subject lines: You need to make them concise, or else they’ll get cut off. Grab your phone and see for yourself. 

So as you brainstorm subject line ideas, aim for 60 characters or less. We also recommend using Zurb’s free testing tool to see how your cold email subject lines appear on different mobile devices.

Short and simple subject lines to try:

• “[Your product benefit in 3-4 words]”
• “[Recipient’s Company] <> [Sender’s Company]”
• “Solution to [problem]”

Do: Personalize it 

There’s a lot of research showing how personalization can drive better cold outreach and email marketing results. It works because unlike ads or SEO content created for large audiences, email is a one-on-one channel. By making prospects feel like individuals rather than just another name on your email list, personalization taps into emotional interest to create a sense of intimacy.  

You don’t have to wait for the body of your email to take advantage of personalization—in one 2019 study, researchers found that personalizing email subject lines led to higher open rates. 

(Remember, of course, that high open rates aren’t the only thing you should optimize for in your cold outreach.)

The simplest way to personalize your subject line is by using the recipient’s name or their company name, but we encourage going a step further whenever possible. 

For instance:

• Mention their most recent blog post, tweet, etc. Make it clear you’re familiar with their work.
• If your recipient has recently changed roles or companies, congratulate them. 
• Refer to a detail in the prospect’s LinkedIn or Twitter profile, like a personal interest or volunteer activity. 

Good personalization will require additional research on your prospects, but it’s well worth the effort. Many prospects will intuit that you’ve done your homework and become more open to reading your message. 

Great examples of personalized email subject lines:

• “Really enjoyed your latest post on [publication name]”
• “Your latest tweet about [topic] made me [reaction]”
• “Congrats on your recent promotion, [Name]”

Do: Establish some kind of connection

Remember that one of the common motivations behind why people decide to read a cold email is because of relational interest. This happens because we’re biased to think more positively of people we feel similar or connected to. 

So despite coming from a stranger, people are usually more compelled to consider a message if they perceive some kind of connection. It makes the prospect feel like they already have a relationship with the sender. 

You can do this in a few ways. For starters, if you have a mutual connection with your prospect, mention them as a referral. Or if you’ve graduated from the same university or attended the same conference, bring it up. 

An important note: You should always establish a connection honestly. Don’t lie about what your mutual connection has said or done, or who you are. Look for something you have in common with your prospect—then work this into your subject line.

A few thoughtful ways to create a connection:

• “[Mutual connection] suggested reaching out to you”
• “Fellow [university] alum here”
• “Missed the chance to talk to you at [event]” 

Do: Pique curiosity about something relevant

Think about your subject lines as cliffhangers. They should create some suspense—this way, they’ll drive your prospects to want to learn more. 

The key is to make sure your subject line relates to your content and/or prospect. Give them just enough to want more. For example, that could be by addressing the recipient’s pain point (the problem you solve) or mentioning the specific type of resource you’re emailing about.

Examples of how to stir curiosity:

• “Advice on [prospect’s pain point]”
• “Interesting case study about [prospect’s industry]”
• “Noticed a broken link on one of your pages”

Do: Make it self-evident 

We’ve found that obvious subject lines often convert well because prospects generally don’t like guessing what your email is about. So by making your message’s subject line clear and to the point, you’re doing prospects a favor.

(That’s not to say you should throw creativity completely out the window, though. It’s worth testing slightly opaque subjects if they include sharp hooks and get people to open and convert.)

One caveat here: Avoid writing self-evident subject lines for follow-up emails, like “Just following up.”

This level of obviousness doesn’t work so well for follow-ups because chances are, your prospect didn’t have much reason to actually open or respond to your first email. They need something more compelling to be inspired into action, like more information.

Obvious subject lines for initial cold emails:

• “[Industry] leads for [Recipient’s Company]”
• “[Recipient’s Company]’s reviews on G2”
• “Question about [Recipient Company]’s marketing KPIs”

Do: Use power words to grab prospects’ attention

Power words are evocative words or phrases that stir emotion. Think descriptive adjectives and verbs like clever, convenient, boost, and unexpected.

The point is to elicit emotional interest—but be careful not to exaggerate or overpromise. Otherwise, you’ll set yourself up to disappoint prospects.

For example, say you’re selling a highly innovative product. You could use a subject line like, “Unheard of way to [the solution your product provides].” 

Or if you have quality research or social proof, you could tease readers with “Record-breaking increases in [relevant metrics].” 

Other ideas for using power words:

• “A creative way to engage [prospect’s target audience]”
• “Our secret to improving [product]” 
• “New tool that can help automate [process]”

Do: Experiment

There’s ultimately no one-size-fits-all answer to writing a winning subject line, especially for readers across different industries. What this means: Rather than stick to a few formulas (including the ones we’ve mentioned above), you should regularly test your subject lines and try new types. 

In fact, slight tweaks to your email subject line can drive major changes in your email open rate. So to get the highest ROI out of your email campaigns:

1. Create multiple subject lines.
2. A/B test them on ~10% of your list. 
3. Set up your email software to send the iteration that receives the highest open rate to the rest of your list.

Starting out, we recommend trying small variations on your initial subject line. We’ve found that these are most cost-effective ones to test because they require less time to write. However, if you’re not seeing any significant changes in your email metrics, come up with drastically different subject lines.

And while the approaches mentioned earlier often produce consistent results, we’ve also seen some success from a few other tactics. They may not work for all audiences, but it’s worth giving them a try:

• Include an emoji. Search Engine Journal tested emojis in its email campaign subject lines over a two-month period—and got unusually mixed results. While non-emoji subject lines drove slightly higher open rates, those with emojis saw significantly more click-throughs. Our take: A big part of whether emojis will work for you depends on your brand reputation and perception—so try testing them only if they don’t clash with your branding. 
• Connect the subject line to your preview text. Some marketers take advantage of preview text (the snippet of text that appears in the prospect’s inbox) by treating it as an extension of their subject line. For instance, an email might use for its subject line “Miss in-person networking?” and then follow up with preview text like, “Look no further—you’re invited to…” This helps when you simply can’t contain your subject line within 60 characters, or you have a particularly great opening line to work with.

• Create a sense of urgency. There’s some research behind the power of FOMO—fear of missing out—in getting prospects to read cold emails. But depending on your execution, this can be a double-edged sword. Done well, it entices prospects to open. But if you overexaggerate your message’s urgency (or it’s clearly not time-sensitive), it’ll come off as clickbaity—and recipients will send your email to their spam folder. 

Subject line ideas worth experimenting with:

• “🎯 How to avoid [problem]”
• “Reaching out with free tickets—but I’m running low”
• “[#] days left until [event]”

Don’t: Use false prefixes or clickbait

The 2003 CAN-SPAM Act requires that subject lines “accurately reflect the content” of your message.

So if this wasn’t already obvious, you should know that false prefixes like “FW” or “RE” aren’t okay. Not to mention, they automatically trigger spam filters. Even if these emails make it to people’s inboxes, they’re often deleted immediately. The same goes for clickbait subject lines.

Since these types of subject lines rely on deceit, you’re doing more harm than good. Email recipients will be more likely to send a spam complaint, which’ll hurt your email deliverability in the long run. 

Examples of subject lines you should never use:

• “Re: Potential collaboration”
• “FW: great news”
• “Make 10x more money”

Don’t: Write vague subject lines

Some marketers write vague subject lines because they think this format piques curiosity. 

For some recipients, they can work in getting some opens. 

But for recipients with busy schedules or those juggling multiple priorities (like executives and senior managers), generic subject lines come off as a waste of time. After all, a subject line like “Got a question for you” doesn’t communicate much value. 

As we suggested earlier, you should intrigue potential customers by giving them a sneak peek of your email content. Don’t leave them completely in the dark. 

Avoid these generic subject lines:

• “Check this out”
• “Quick question”
• “Website inquiry”

Don’t: Ask for something without offering anything in return

There’s something to be said for straightforward and direct wording. The whole point of doing email outreach, after all, is to drive some kind of action from your prospects. 

Still, you shouldn’t ask for anything outright, especially not without giving some background or context first. Most people can’t be bothered to do anything for a stranger if there’s no obvious benefit for themselves.

Avoid these:

• “Let’s hop on the phone”
• “Request to connect”
• “I need a post on your website”

Don’t: Use all caps or a lot of exclamation marks

Like false prefixes, writing subject lines in all caps can trigger spam filters.

For instance, this cold PR email landed in spam despite providing valuable information (about Girl Scout Cookies!).

Exclamation points create the same issue. In a study of more than 115 million emails, emails with an exclamation mark in their subject line received fewer opens than those without one—with an average open rate of 45.% compared to 51.9%.

Subject lines that send your email to spam:

• “URGENT RESPONSE NEEDED”
• “COMING SOON: This will change your life!”
• “The perfect solution for [Recipient’s Company]!!!”

Don’t: Write very formally or use overly promotional wording

Take a look at these two cold emails from the PR world, both from firms reaching out to promote their respective clients. 

Which is easier to understand? 

If you’re a car enthusiast, you might’ve recognized the names VinFast (a new automobile startup brand) and Pininfarina (an old car design company). But to unfamiliar readers, that first subject line makes little sense. (The terms “VF 8 and VF 9” hardly help give more info.)

The second email, on the other hand, is obviously about running advice.

Yes, the two email topics are apples and oranges, but there’s a clear difference in readability. The second email feels a lot more accessible because it uses relatable language.  

The takeaway here: keep your subject lines casual. Don’t make them too formal or salesy. And avoid jargon. 

Prospects don’t want just another sales email in their inbox, least of all from a stranger. Write politely but conversationally, like how you’d speak to a new friend. 

Examples of unapproachable subject lines:

• “Super Sale! Today Only—Act Now!!”
• “Dearest Madam”
• “DTC marcomm automation [and any jargon-heavy phrases]”

Wrap-up

The best cold email subject lines intrigue and pique readers’ interest. They don’t mislead or overpromise. Instead, they tend to be clear and easy to understand, and they often use the prospect’s name. 

Still, even with these guidelines, subject lines aren’t easy—and it can feel unfair that well-written, relevant email copy will be ignored based on them.

While there’s no surefire way to get all recipients to open your cold email, you can always work on optimizing and improving your subject lines. Remember to experiment and try new copywriting ideas. That might even mean breaking some of the rules above. The point is to test different subject lines and then iterate on the versions that resonate with your audience.