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In cold outreach, one of the most impactful metrics is open rate. Email deliverability, sender name, even the email preview will impact your performance, but no element has quite as much pull as the subject line. It is no wonder that 47% of recipients open an email by looking at the subject line alone. 

The plethora of mediocre subject lines in your mailbox is due to most businesses making common mistakes and lacking insight into cold email strategies. Today, I am sharing with you 16 subject line insider tips that have helped us and our customers get 50%+ open rates so that you can apply them to your campaigns as well. Keep in mind that all 16 tips are not applicable at the same time and depend on your audience and topic.

1. Keep your subject line short and mobile-friendly

What is the point of writing a catchy subject line only for it to get cut off because it is too long? While 40-70 characters are displayed in the subject line on desktops and tablets, mobile phones only show 30-60 characters. I recommend keeping it around 40 characters to ensure it is readable on all devices. 

Also, when a prospect receives your email, the first few words of the email message are displayed in their inbox. Desktop and tablet screens will impact how much of the message appears because it is in the same row as the subject line, as shown below.

Either way, approach writing the subject line by starting with the words that matter most and removing fluffy details.

Good subject line: + hubsell – how about we team up

2. Ask compelling, open-ended questions in subject line

There is no better way to create an instant dialogue than by asking a question. Your recipient’s curiosity peaks when the subject line is left open-ended. Think of it like a cliffhanger that needs answers, just keep your question relevant to their buyer persona. 

You could make your prospect feel useful by asking for their opinion or enticing them to consider their own needs and desires. Just remember that close-ended questions do not have the same effect, as a simple yes/no is not the best conversation starter. 

Questions using emotion are also a winning combination because some emotions motivate people to take action, and it secretly drives most purchase decisions. Awe, relief, amusement, challenge, and joy are great emotions to trigger and if done right will generate tremendous increases in open rates.

Good subject line: What is next for the B2B sales industry?

3. Create a sense of urgency and exclusivity with your subject line

It is a natural human inclination to want to be included in upcoming events and offers, especially if it is a one-time offer. No wonder subject lines that create a sense of urgency have a 22% higher open rate

FOMO which stands for fear of missing out is something that 56% of people experience. This approach can be used in your subject lines by including a deadline or using words such as ‘there is still time’. Injecting FOMO triggers into your subject line will 1) increase open rates and conversions and 2) generate reactionary purchases when social proof is offered. 

You can also use scarcity to motivate prospects to make a purchase quicker because your product or service will appear exclusive. Simply keep in mind that these strategies should only be used if and when you can deliver on them because false claims negatively impact your brand’s reputation.

Good subject line: Want to boost your conversion rate by 25%? Find out how at our webinar event

4. Ensure that you are not using any email spam trigger words

When the words or phrases in a subject line are demanding or overpromise an outcome, spam filters step in to block the email from entering a prospects’ inbox. What is being flagged are words that appear malicious or fraudulent. Choosing your words wisely will ensure that spam filters do not get triggered. 

In your pursuit of effective, spam free email subject lines, here is a list of common words and phrases to avoid. Phrases such as ‘do it today’, ‘buy direct’, ‘limited time’, ‘apply now’, ‘act now’, create unnecessary pressure. Using ‘risk-free’, ‘instant access’, ‘1000x better than’ are keywords associated with overselling and exaggeration. ‘Cheap’, ‘credit’, ‘free’, ‘bargain’, ‘money back’, and ‘save up to’ are also general financial keywords that get flagged. In addition, avoid common keywords associated with employment such as ‘earn extra cash’, make $’, ‘double your’ and ‘be your own boss’. 

Ultimately, the above words and phrases can make people feel hesitant and trigger spam filters because you are pushing for access to someone’s personal information. A general rule is to stay clear of making bold claims that you can not deliver on as this does the opposite of optimising your email open rates.

Bad subject line: Grow your lead generation at 1000x speed with outbound sales risk-free.

5. Start by drafting 3-5 possible subject lines for each email

According to Upworthy, a digital media company, employees should think of at least 25 headline options for every article. The same tactic should be applied to email subject lines because they are usually an afterthought. The aim behind this challenge is to eliminate weaker ideas and allow noteworthy ones to surface.

You do not have to create 25 different types of subject lines. Start with 3 and experiment with questions, negative, positive, and neutral tone of voice, personalise some, and add humor to others. Weed out subject lines until you are left with the two best ones and turn to A/B testing to learn which performs better. The results will eventually condition you to quickly determine which alternative works better. 

Example Topic: How to get higher open rates on your email

Subject 1: higher open rates

  • Super matter-of-fact subject just explaining what is inside.

Subject 2: I tripled email opens on active campaigns like

  • Use an example result.

Subject 3: my email opens went from 14% to 39% with hubsell

  • Use a number.

Subject 4: Wow my email open rates just shot up

  • Be fun and a bit wacky.

Subject 5: what is the best way to get higher open rates?

  • Pose a question

6. Personalise the subject line beyond static placeholders

Personalising your subject lines in automated outreach messages is a great way to get higher open rates. In fact, people look out for subject lines that reflect their interests and behaviour. Something as simple as adding a static placeholder which is your prospects could make them feel special.  

Do not stop there. There is another level of depth to add to your subject lines called dynamic placeholders. Examples include adding a prospect’s gender, industry, title, and months in current position. 

These placeholders improve your subject  line in two ways: :

  1. Make your subject lines 100% customised to optimise your conversion rates
  2. Prevent emails that exceed the daily limit of 100 emails per mailbox, from triggering spam. 

Due to popular demand, hubsell recently incorporated this feature in our software and it has dramatically improved open rates for our customers.

Good subject line: (If prospect is in sales) + hubsell – business leads and sales opportunities

7. Use action-orientated verbs to inspire your audience to take action

Email subject lines and calls to action (CTA) have one thing in common, they both use words and phrases to entice a prospect to click and take action. The best way to encourage your prospect to act is by using actionable verbs such as register, learn or download. 

This approach is a double whammy, as you are also speaking to your recipient with an active voice instead of a passive voice, which will make them feel like you are talking directly to them. 

Also, when I come across action-orientated verbs at the beginning of a subject line, I instantly feel a sense of urgency to act and have clarity about what the sender is asking me to do.

Good subject line: Increase your open rates by 50%+ by trying our 6 tips

8. Stay clear of tricks and clickbait strategies

Using tricks and clickbait subject lines such as, ‘You will not believe this shocking secret’ is a dramatic and over-sensationalised tactic used to convince a recipient to open an email that often does not deliver what was promised. 

These tactics may shock your prospect into opening the email once or twice, but eventually, you will lose their trust and your brand will be viewed in a negative light. 

I recommend staying away from common tricks such as using ‘fwd’ or “Re:” in your subject line to give the impression that the recipient already knows you. Deception like this can seriously damage your subscriber list, and your ability to convert leads. 

Bad subject line: Grow your lead generation by 2000% with outbound sales in 2 weeks

9. Use logical keywords for search and filtering purposes

Are you ever too busy to read an entire email when you receive it and later find yourself searching endlessly through your inbox to locate it? 

Omitting important keywords leads to your subject line not popping up when your recipient applies filters and will agitate anyone when designating different topic folders. You can easily avoid getting into this situation by including keywords in the subject line that relates to the topic of your email. 

What keywords you use and how often you apply them are also important. For instance, some keywords make recipients feel rushed such as “fast” and “act now”. Also, the keyword “update” features in many well-performing campaigns, but overuse will not have the same effect. 

Good subject line: The 17 best subject lines for your next email marketing campaign

10. A/B test all of your subject lines to drive better results

A great way to analyse which subject lines are going to get you the open rates you want is to test multiple options using A/B testing tools. At the beginning of your outbound campaign journey when you have a low email sending quota, A/B testing may not be pivotal as of yet. You will get to a point though where you can optimise your results by testing which subject lines encourage engagement. 

The trick is to only alter one variable of the subject line at a time, so you are able to track what actually worked better. Test variables such as short vs long formats, a positive vs a negative approach, or curiosity-provoking vs specific. Also, running the campaign long enough will enable you to analyse the effects. 

Good A/B test: + hubsell – how about we team up


, Increase ROI through outbound techniques with hubsell

11. Bring joy into your subject line by adding some humour

Making your prospect smile or laugh is always advantageous especially when their email inboxes are filled daily with ordinary subject lines. Pique your prospects’ interest by using humour and bringing joy even if your brand is a serious one. 

The trick is to know your audience. If you are emailing a potential business partner or CEO for the first time, you might not want to lead with humour, but if you are following up with a prospect that you had been in contact with for a few weeks, then this could reinvigorate the conversation.  

Try including puns, rhymes, alterations, quotes from movies and songs, and connect your product to something out of the ordinary.

Good subject line: GDPR = Gosh Darn Pretzel Rolls to sweeten up your data sets

12. Use numbers, symbols, and punctuation appropriately

Similar to blogs, some numbers and symbols can be an effective marketing practice if used appropriately. 

Numbers that are relevant to the actual email copy such as a discount offered, referring to the specific number of tips or the title of your listicle will appeal to prospects who 1) do not have a lot of time to read or 2) prefer getting information in small chunks that are easily digestible.

Symbols like // and – and () can be used to separate and clarify the subject, but others like * ~ ^ % $ # should not be used because they generally trigger spam filters. As for multiple exclamation marks and ALL CAPS, these are sure-fire ways to come across like you are yelling at your prospect and seem unprofessional. 

Bad subject line: “APPLY NOW FOR A FREE TRIAL” 

13. Make people so curious with your subject line they instantly click

Humans are inherently curious when there is an information gap between what we know and what we wish to know. Your email subject line could be this teaser that provokes your prospect to the point that they cannot not open your email. 

By omitting just enough information in your email subject line your recipient will become curious and seek the missing information by reading further. It is however important to understand the difference between curiosity and clickbait. 

Here is a great example ‘new to CRM systems? Here is what you need to know. This subject line focuses on what the recipient does not know and entices them to fill this information gap by reading the email. To avoid this strategy from backfiring and becoming clickbait, it is important to offer closure in the email copy and bridge the information gap. In this case, the email copy should offer valuable insights into what a CRM system is, different CRM systems to choose from, or how to optimise them. 

Good subject line: Can’t decide what CRM system to use? This will help

14. Check for spelling or grammatical mistakes before sending

After years of receiving email campaigns from salespeople, I have seen all, from great, mediocre to shocking subject lines. 

Subject lines loaded with grammar or spelling mistakes are a common occurrence and lead me to think either 1) the sender is unable to make a good first impression and perhaps a negligent business partner, or 2) the sender’s writing and language skills do not match mine, which open doors for miscommunication.

To optimise your email open rates and make a good first impression, I recommend rereading and spell-checking your subject line before running a campaign to ensure you look professional and prepared. 

Bad subject line: Grow you’re lead generation with hubsell

15. Be specific and share something useful with your audience

How many emails stand  unopened in your inbox because they have zero meaning to you? Office workers receive on average 121 emails a day, meaning that your recipients prioritise which emails to open based on relevance and curiosity. 

Before sending any campaign, it is important to confirm whether your subject line highlights a particular benefit and if it is immediately clear why your email is useful to the recipient. 

You can actually provoke curiosity and be specific in the same subject line. Below is a subject line that is focused on a specific company’s search performance, while also implying the usefulness of gaining first page rankings. 

Good subject line: Why X company’s blogs do not feature (on Google’s first page of search results)


16. Show that you are an expert in your field with your subject line

Earlier I pointed to the benefit of including social proof in your email subject line, well the same can be said for your expertise and authority in your industry. 

Prospects will respond and join your mailing list when they 1) feel that they have gained knowledge that they would otherwise not have known or 2) they admire your business and want to gain more information. There is no better place to set the tone and distinguish yourself than in your subject line.

Good subject line: How we optimised our intent data using


In this article, I unpacked the best practices to help salespeople angle their cold email subject lines the right way with the aim of increasing open rates and boosting conversions. A good email subject line is one that creates a genuine connection, provokes curiosity, is brief, or offers value. The most effective email subject lines are those that use a combination of strategies. 

Bulletproof your email campaigns by using these subject line strategies in tandem with our tried and tested email templates that offer structures for the body of your email, and can be used in any situation. 

New readers

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