People may or may not judge a book by its cover, but they do judge an email by its subject line. A subject line for cold emails is the first impression of you as a marketer/seller.  

According to statistics, 33% of email recipients open emails depending on the subject line, and adding personalized information is likely to increase open rates by 50%. 

Bottomline—subject lines are crucial for cold emailing, and crafting one can be trickier than you think. 

This article contains 15 cold email subject lines DOs and DON’Ts. 

Let’s dive in. 

15 Cold Email Subject Line Do's and Don'ts

If you’re wondering how to write better subject lines, here are 15 dos and don'ts of cold email subject lines to consider. 

Things you need to DO to craft the best subject line for cold email campaigns:

1. Keep subject lines text-based
2. Keep it within 30 to 45 characters (4 to 6 words)
3. Personalize
4. Always conduct A/B testing
5. Refer to pain point
6. Provide a hint about the email body
7. Add a sense of urgency
8. Use power words

Things you should NOT do while crafting a subject line for cold email campaigns:

1. Don’t use sales-y language
2. Don’t use all CAPS
3. Don’t ask for a favor
4. Don’t use emojis
5. Don’t use special characters and too many numbers 
6. Don’t mislead recipients
7. Don’t use spammy words

Read this article till the end to learn about the above points in detail. 

8 Do's of Cold Email Subject Line 

Consider the below points as email subject lines best practices. Also, note that experimenting is the way to ace outbound sales. So, always, always conduct A/B testing to find what works and what doesn’t for your target customers. 

Find a tool like Smartlead that comes with cold email A/B testing features, and you’re good to go. 

Let’s dive into the subject line crafting best practices. 

#1 Keep subject lines text-based 

Using special characters or excessive numbers in cold email subject lines can trigger spam filters because they resemble common tactics used by spammers. Spam filters often interpret such elements as attempts to manipulate or deceive recipients. 

To maintain authenticity and improve email deliverability, it's crucial to keep subject lines text-based, avoiding tactics that mimic spam patterns. This ensures that legitimate emails are more likely to reach recipients' inboxes and enhances the overall effectiveness of cold email campaigns.

#2 Keep it within 30 to 45 characters (4 to 6 words)

Keeping cold email subject lines within 30 to 45 characters (4 to 6 words) is beneficial as short lines grab attention quickly, increasing open rates. They're mobile-friendly, ensuring easy readability on small screens. Examples: 

✅ Brandon, up for 30% reply rate?

✅ Jon, check this out

✅ Secret behind Peakora’s 40% reply rate

According to stats, xy% of users open emails through mobile phones. Using short subject lines makes your email mobile-friendly, increasing your chances of a higher open rate.

These concise lines avoid beating around the bush and express the intention behind the email directly, enticing recipients to open emails promptly. 

#3 Personalize

Personalizing email subject lines is vital as it increases engagement by making the subject line relevant to the recipients. Customization can involve using the

➡️ recipient's name

➡️ referencing specific details about their company

➡️ tailoring the message to their interests. 

Including relevant information in the subject line establishes a connection, increasing the likelihood of the recipient opening the email. This personal touch creates a more meaningful and compelling first impression, ultimately leading to higher email open rates and a better chance of achieving the desired response in cold email outreach.

#4 Always A/B Test 

A/B testing involves comparing two versions (A and B) of a variable of an email, like a subject line or opening line, to determine which performs better. 

In cold email campaigns, A/B testing is crucial to optimize email subject lines for maximum engagement. It helps identify the most effective language, length, or personalization, refining strategies based on real-time data. 

Go for cold email tools like SmartLead that come with A/B testing features as well as cold email subject line analyser. Along with A/B testing, Smartlead also offers unlimited warm-ups, mailboxes, and a master inbox. This comprehensive solution enhances efficiency, ensuring your cold email campaigns are not only personalized but also backed by data-driven insights for continual improvement.

#5 Refer to the pain point

Referring to recipients' pain points in cold email subject lines is one of the most important dos of cold email subject lines, as it immediately captures their attention and relevance. Examples:

• "Solve Workflow Woes with {Your Solution}"
• "Boost Efficiency: Conquer Data Overload"

By addressing challenges or concerns specific to them, you demonstrate an understanding of their needs, establishing a connection. This empathetic approach makes your email more compelling and increases the likelihood of the recipient engaging with the content. 

Focusing on pain points establishes a common ground, showcasing your solution as a valuable resource tailored to address their specific issues, ultimately improving the effectiveness of your cold email outreach.

#6 Provide hint about the email content

The best email subject lines provide a hint of the email's content to set clear expectations and entice recipients to open the message. Recipients are more likely to perceive it as relevant and worth their time when the subject line offers a glimpse into the email's purpose or value proposition. 

This transparency establishes trust and encourages engagement, as recipients appreciate knowing what to expect. Crafting subject lines that align with the email's content improves open rates and contributes to a positive recipient experience in cold email campaigns.

#7 Add a sense of urgency

Adding a sense of urgency in subject lines is crucial because it prompts immediate attention and action. Time-sensitive language compels recipients to open the email promptly, fearing they might miss out on a valuable opportunity or limited-time offer. Examples:

• "Limited-Time: Unleash Productivity Power Now"
• "Act Fast: Exclusive Software Upgrade Available"

This psychological trigger creates a heightened interest, increasing the likelihood of engagement. By emphasizing urgency, you create a compelling reason for recipients to act swiftly, driving the desired response and improving the overall effectiveness of your cold email campaigns.

#8 Use power words

Power words are persuasive and emotionally charged terms that evoke strong responses. Incorporating them into cold email subject lines is essential to capture attention, evoke curiosity, and drive action. 

Power words trigger emotions, making your message more compelling and memorable. 

Subject line examples with power words are:

• Create lead lists in 3 clicks only
• Revolunatize cold email game with master inbox

7 Don'ts of Cold Email Subject Line 

If there’s a list of cold email subject line best practices, it would be filled with DON’Ts. One can craft the perfect subject by only focusing on the below points. 

From warning against emojis to spammy words, the below listed DON’Ts of subject lines will help you ace the cold emailing game. 

#1 Don’t use sales-y language

Sales-y language in a cold email refers to pushy, overly promotional, or aggressive wording that focuses solely on selling a product or service. Using such language can lower open and reply rates because recipients may perceive it as insincere or spammy. 

❌ Buy this {tool} to increase ROI by 40%

❌ Don't Miss Out: Unbeatable Software Savings

Additionally, it can increase bounce and spam rates as spam filters often flag emails with overtly promotional content. To improve email deliverability and engagement, it's crucial to strike a balance, using language that is personalized, relevant, and focused on providing value to the recipient.

#2 Don’t use all CAPS

Using all caps in subject lines can harm engagement metrics. While it may initially grab attention, all caps can be perceived as aggressive or spammy, leading to lower open and reply rates. Recipients read subject lines in all caps in a “yelling” tone. 

Additionally, email providers may interpret it as spam, increasing bounce and spam rates. Modern email filters prioritize user experience, and excessive use of all caps is a common characteristic of spam, potentially causing your emails to be flagged or sent to spam folders, reducing overall deliverability.

#3 Don’t ask for a favor

Asking for a favor in a cold email subject line can be off-putting and may lead to lower open rates. Strangers are more receptive to initiating conversations or providing value rather than being immediately asked for assistance. 

❌ Can you help me with a review?

❌  I need your help

Marketers should focus on building relationships by offering insights, solutions, or relevant content, creating a foundation for meaningful interactions. This approach fosters a positive impression and increases the likelihood of recipients engaging with the email, ultimately leading to more fruitful long-term connections.

#4 Don’t use emojis

Using emojis in B2B email subject lines may appear unprofessional and diminish the credibility of the message. Emojis are often associated with casual communication and might be perceived as gimmicky or unprofessional in a B2B context. 

In business communications, maintaining a balance between a formal and serious tone is crucial for establishing trust and professionalism. Emojis could potentially be misinterpreted or not align with the professional expectations of B2B recipients, making it advisable to avoid them in cold email subject lines targeting the B2B space.

#5 Don’t use special characters and too many numbers 

Avoiding special characters and excessive numbers in subject lines is crucial to prevent triggering spam filters. Many spam detection algorithms identify these elements as common tactics used by spammers to manipulate or deceive recipients. 

❌ Unlock Exclusive Discounts Now!!!

❌ Increase ROI by 50% - Act Fast!!!

Using too many numbers or special characters can lead to your emails being marked as spam, reducing deliverability. Keeping subject lines clean and text-based ensures that your emails are more likely to reach recipients' inboxes, improving the chances of engagement and avoiding the negative impact of spam filtering.

#6 Don’t mislead recipients about the email content

Emailing recipients with clickbait subject lines can damage trust, credibility, and brand reputation. Clickbait is misleading and often leads to disappointment when the content doesn't fulfill the promised value. Over time, this erodes the trust of your audience, leading to disengagement or even unsubscribes. 

Building authentic relationships is crucial in cold email marketing, and using clickbait can undermine this trust, negatively impacting your long-term relationship with your audience. For sustainable engagement, it's essential to maintain transparency and deliver on the expectations set by your subject lines.

#7 Don’t use spammy words

Avoiding spammy words in subject lines is crucial to prevent emails from being flagged by spam filters and to maintain credibility with recipients. Spammy words trigger spam filters and may lead to lower deliverability rates. Some of the examples of spammy words are "guarantee," "free," "earn money fast," "limited time," and excessive use of exclamation marks. 

You can increase the chances of your emails reaching recipients' inboxes and foster trust with your audience by avoiding such spammy words. 

Final Thoughts

A short, personalized subject line that does not include spammy words but touches the recipient's pain point is ideal for cold email campaigns. It is tricky to get it perfect in one go. 

I recommend following the above DOs and DON’Ts of cold email subject lines. To find the perfect subject line, I suggest doing A/B tests.

A/B testing various subject lines or different versions of the same sentence will help you understand what works and what doesn’t with your target audience. Check out Smartlead for an easy A/B testing feature.