14 min
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August 3, 2023

Deep Dive: Writing great email subject lines (AI prompts + examples)

Josh @ Hoppy Copy
3x founder, CMO, loves email marketing
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Harsh fact – It doesn’t matter how fantastic or life-changing your email is… if nobody opens it.

You’ve got 3 seconds.

That’s it. Three seconds to catch the reader’s attention in their inbox. And we should hit 350 billion (with a B) emails sent every day sometime this year.

The good news is you don’t have to compete with all of them. The bad news is you’re still competing with around 120 emails the average person gets every day.

So your subject line has to be good.

And if it’s not, you’re just burning money. And your hard work crafting your email is going to waste.

So yeah. No pressure.

But don’t get too stressed about it. We’ll take you on a deep dive into what it takes to write great subject lines.

Let’s start with some quick and easy basics...

Subject line basics

Your readers expect you to get this stuff right, and you probably are. But then again, are you certain? Taking a second look is never a bad idea. So, let’s check out your basics.

Customize your From field with care.

I know this isn’t part of the subject line, but 55% of people decide if they will open the email based on who it’s from. And you would be surprised how often even huge companies get this wrong.

Make sure the From field identifies who the email is coming from. It’s ok to test between the company name and joe@company name or similar variations but keep it consistently identifiable.

All of these are from the same company. You can see why I didn’t recognize a couple of them and almost reported them as spam.

Subject line examples - bad From field

Keep the length to 50 characters (including spaces) or less

Half of email users are now opening them on mobile devices, so it’s more and more essential to keep it short. On my phone, the notification shows 43 characters, and the app shows 53 characters in the subject line. If you must go longer, at least put the important information at the beginning of the subject line or in your preview text.

My desktop                 vs.                   my phone

Subject line examples - length of space

Match your tone and level of formality to your brand.

Your tone is the feel of your brand and message - what your audience associates with you. It can be casual and fun, serious and more formal, or anything in between. You can play with both tone and formality to see what resonates most with your audience.

B2B tends to be more formal, while B2C prefers informal. But that is a generalization.

Your readers already responded to how you talk to them. Start with small variations of your current tone to see what works best.

If you aren’t sure what tone to use, check this article out.

Use sentence case in your subject lines

You can use capitalization for emphasis, just don’t get carried away.

subject line example - capitalization for emphasis

Of course, don’t use all caps unless you mean to yell at your readers.

subject line example - all caps

Don’t use the title case, either. The reader will assume you’re trying to sell them something. Audiences prefer to read something that looks like it came from a person rather than a business.

subject line example - Title Case

You might experiment with all lowercase. This comes across more like an email from a friend.

subject line example - all lower case

To emoji or not to emoji

The data is about 50/50 on this one. If you only use them occasionally and appropriately (meaning that they match the context of the subject line), then go for it. 🙂 They do draw attention. I bet you caught both in the examples above.

Personalization is becoming a must-do thing.

Yes, it goes way beyond subject lines, but we’re just talking about adding your reader’s name to the subject line right now. ‘Hey, Bob’ comes across much better than ‘Dear Sir or Madam’. In fact, adding their name to the subject line can increase open rates by 26%.

subject line example - using reader's name 1
subject line example - using reader's name 2

Segment your list

This is the other part of personalization in subject lines. You want to send the right information to the right people at the right time. The easiest way to do that is through segmentation.

You can segment your list by demographics, behavior, interests, or when they signed up. Your email marketing provider should have built-in capabilities for this. Segmenting allows you to tailor your approach to the needs and interests of the groups. You’ll get much better engagement and better conversions.

Creativity vs. Clarity – Don’t sacrifice one for the other

Don’t get so creative that you sacrifice clarity. Readers don’t like to be confused and don’t want to read about something they aren’t interested in. So it’s best to be clear and not risk alienating your audience. Angie Colee is a top copywriter, and I’m sure this got some curiosity opens. I didn’t open it because it didn’t say anything to me.

subject line example - creativity over clarity

On the flip side, don’t sacrifice creativity by only using boring subject lines. This example tells you what is in the email but doesn’t give you any reason to want to read it.

subject line example - clarity over creativity

There is a lot of space in between the two extremes where you can play and create really great ones.

subject line example - creative and clear

Words to avoid

Spam filters try to catch anything reported as fraudulent or malicious. The most recent list I’ve seen is 394 words and phrases. In general, avoid anything that is overly hyped, sensationalized, or promises that are too big. If you want your readers to trust you, don’t say unbelievable things.

Also, avoid using words that sound salesy (like ‘buy’ or ‘attention’ - even if you’re talking about getting it).

Don’t use any that indicate difficulty or require effort from the reader (like ‘study’, ‘work’, or ‘hard’}. And words that are culturally negative {like ‘ego’}, or unpleasant {like ‘wait’). Who wants to read about those?

No clickbait

Make sure the content of your email matches your subject line. Your readers will stop opening your emails if they don’t get what they expect.

subject line example - hype / clickbait

I didn’t get any additional information in the email, just more hype and a link to find out more.

subject line example - body copy that doesn't deliver

I followed the link, and after 15 minutes of more hype, I gave up and closed it. I never found out what herb they were talking about.

I’ve had the same experience with another of their emails. I won’t open another one. They did a lot of work to make me not want to buy their product.

Subject line formulas
(with templates)

The subject line’s job is to get your reader to open the email, and make sure it stands out from all the others in their inbox.

Don’t use the same type of subject line every time. Anything done too often will become boring and get fewer opens.

Create a bunch of subject lines for each email, and then pick the best one.

Benefit – Tell the reader what is in it for them if they read the email,

  • Get {benefit} in {X} days – Get clear skin in 7 days
  • Here is the {benefit} you requested – Here is the ebook you requested
  • How to get {benefit} from your {person} – How to get respectful language from your teenager

Intrigue – Use their curiosity to get them to open the email.

  • You won’t believe {thing} – You won’t believe what Jeff Walker is doing next.
  • What does {thing} have in common with {subject}? – What does string cheese have to do with email marketing?
  • One {thing} stands between you and {situation} – One word stands between you and success.

Announcement/Urgency – showcase new products, offers, or other news

  • Your {offer} expires in {timeframe} – Your exclusive membership offer expires in 2 days
  • New [product] does [action] twice as fast as before – New pen writes twice as fast as before
  • [Holiday] sale [discount] [X] day only – Labor Day 25% off sale, one day only

List – great for how-to and product emails

  • [X] ways to [thing] – 7 ways to improve your sleep
  • [X] easy steps to [thing] – 3 easy steps to financial freedom
  • [X] favorite [products] in our store – 25 favorite T-shirts in our store

News – boost your credibility with industry-related topics

  • [thing] shows [result] – Harvard study shows adults prefer peanut butter in their chocolate
  • [media outlet] says [news] – Google says email marketing is the king of ROI
  • [company] voted best [type] in [place] – Ivy’s Igloos voted best affordable housing in Arizona

Question – people want to know the answers, so they will keep reading.

  • Are you making this [type] of mistake? – Are you making this illegal bookkeeping mistake?
  • Can you [do something]? – Can you answer this question?
  • So you know [something]? – Do you know why mint flavors feel cold?

Personal – If you’re collecting the data to do this, it shows your readers that they aren’t just another potential sale.

  • [occasion], [name]! [gift] inside – Happy Birthday, Stacy! 30% off coupon inside
  • You bought [product] [X} weeks ago. How is it going? – You bought our P45Q program 4 weeks ago. How is it going?
  • [product] is back in stock – Our Dog Translating Collar is back in stock!

Juxtaposition – Use two ideas that normally wouldn’t go together. It stands out and makes people want to know more.

  • [small timeframe] that ruined my [large timeframe] career – 5 minutes that ruined my 25 year career
  • From [descriptive] to [different descriptive] – From the worst mom to Mother Magazine’s top parenting coach
  • How [type of person] learned to be [different type] – How this beach bum learned to become the CEO of a Fortune 500 company

Tips and examples for standing out from the crowd

  • Use puns or wordplay – "Say Goodnight to Insomnia" (Harvard Medical School)
  • Use numbers or statistics – "Daily miscommunication affects 66% of business leaders" {Grammarly)
  • Make a bold statement – "my best biz decision of 2022 (+ 2023)!" (Susie Moore)
  • Address a pain point – "Does Your Dog Have Too Much Energy?." (Modern Dog Magazine)
  • Appeal to emotions – "What to do when you’re always failing, burned out, just want to quit" (Ben Settle)
  • Use sensory language – "What's causing your itching, teary eyes?" (Harvard Medical School)
  • Offer a solution – "How to Overcome Disorganized Attachment." (MedCircle)
  • Use social proof – "People in United States follow these creators" (LinkedIn)
  • Include a cliffhanger – "It could happen any day now...." (StandoutOnlineSystem.com)
  • Make it exclusive – "Serious players only" (Katrina Ruth)
  • Use FOMO (fear of missing out) – "Are you missing out on FREE grooming visits? " (Petco)
  • Appeal to their interests – "For Creators. By Creators." (Pet Age)
  • Be weird — ”Clown feet ” {Laura Belgray)

How will you know if the crowd is doing the same thing you’re doing? Easy. Use Hoppy Copy’s Monitoring tool. You can track your competition to make sure your emails look different. And you can find tons of inspiration from other marketers that are killing it.

How to use Hoppy Copy to get great subject lines

We’ve created some great templates to help you with everything email. Subject lines included.

Use one of our templates.

There are two tools for email subject lines. Click on AI Wizard and scroll down until you see Write Subject Line for Email (2nd down on the right), and Rewrite Subject Line (2nd down on the left).

If you click on Write Subject Line for Email, you will see this box:

Enter the body of the email and click Create copy. You’ll get 5 options to choose from.

results of the write subject lines for email tool - 5 options

And if you don’t like any of them, just hit Create copy again, and you’ll get 5 more.

If you click on Rewrite Subject Line, you will see this box:

AI - Rewrite subject line tool

Type in a subject line you want to work with, click Create copy, and you’ll get 5 variations on that headline.

results from the rewrite subject line tool - 5 options

Between these two template options, you should be able to craft the perfect subject line.

Or, use prompts in the chat function.

Hoppy Copy gives you another way to create subject lines. Go to the Chat function on the left sidebar.

AI Chat tool with 10 subject line examples

I wrote, “write 10 email subject lines about email subject lines”. You can see what it gave me.

Other prompts you might use are:

  • Write 20 funny email subject lines about puppies.
  • Pretend you are a lawyer and write 10 email subject lines about traffic fines.
  • Write 5 email subject lines about cheese. Use statistics.
  • Write 7 email subject lines that talk about back pain.

Have fun and play around with the AI Wizard. See what you can come up with.


The more information you can give the AI, the better your results will be. And the more you use it, the better you will get at giving it directions.

Always check facts and statistics. There may be more up-to-date information or a better resource to pull from that an AI doesn’t have access to. Maybe.

Testing and optimizing your email subject lines

Marketing is a science as well as an art. Creativity is the art part. The science? That’s testing.

You can’t improve your marketing if you don’t know what’s working and what’s not. And unless you’re running an I-don’t-want-to-make-any-profits business, you need to test and optimize based on those results.

Every top marketer tests each part of their marketing campaigns. Subject lines are just the beginning. It’s easy to do, and it’s essential for your business.

How to test

Use A/B testing (also called split testing) to find out which subject lines perform the best. To do this, choose two versions of your subject line and send them to a small sample of your list (using the same email body content) – one to half the sample group and the second to the other half. See which gets higher metrics. You can send the one that performs better to the rest of your list.

Common testing mistakes

  • Not testing at all. You are missing opportunities if you don’t.
  • Bad formatting. Don’t forget half of your audience read emails on their phone.
  • Too many variables. This makes it impossible to pinpoint what is working.
  • Too small of a sample size. Your results may not accurately reflect your audience.
  • Making changes too quickly. Give your audience enough time to see the email.
  • Not segmenting your audience. Not everyone on your list will react the same.
  • Not tracking your metrics. How would you know what to test or optimize?
  • Not analyzing your results. Again, pointless if you aren’t going to look at the results.

Number of variables to test and which variables?

If you’re just starting the process, test completely different subject lines. Make sure they relate to the body copy, always.

Then as you get more of an idea of what your readers want, test one variable per test. That could be the word order, a statement vs. a question, specific words, the length, with or without an emoji, using the reader’s name, tone, text formatting, or capitalization.

As you get more data, you can refine the variable.

How long should a test be?

Common practice is 24 hours, but how long you run the test is not the critical part. If you get significant data in the first hour or if it takes a week, the data is what matters. For example, if your list is large, in several time zones, or behaves differently depending on the day – your test might take days.

Give it enough time to get an accurate read on your audience’s preferences.

How often should you test?

It depends on your list size and the frequency you send emails. If you’re just starting with email marketing, you can get away with testing only about once a month. Stay consistent to your brand, but start with different styles of subject lines.

Experiment and find out your reader’s likes and dislikes.

As your list gets bigger, you can begin testing once a week.  Once your list gets big enough, you can test each email before you roll it out to your entire list.

You and your readers are unique. Do what works best for you.

If your audience gets overwhelmed with emails or the testing interferes with your marketing plan, slow down a little. It’s really just one big experiment, after all.

Which metric should you track?

For testing, your Open rate will give you the most information on your email subject line performance. But…you HAVE to look at your conversion rate too… because, bottom line, if it isn’t working to make more sales - then it isn’t working.

Your metrics tell you a story.  Make it a success story.

What should you do after the test?

Once you’ve got results you are confident in, use that feedback when you write future emails. Your emails will show improved metrics as you get more and more results. Refining your marketing approach is an ongoing process.

What is the easiest way to do this? Are there tools available?

The easiest way to run the test is to use the built-in options from your email marketing service. They should have the functionality, and you’re already paying for it. Or you could set it up manually.

There are tons of tools available for scoring your subject line.  Here are some good ones:

Subjectline.com (free) – Gives a score and explains what was good and bad about your subject line.  It also has a beta feature that gives alternatives you could use.

Headline Analyzer by CoSchedule (free) – Yes, it was made for headlines but it works just as well for email subject lines. It gives an overall score and improvement tips.

Hemingway (free for desktop interface or $19.99 for the app) – gives you a readability score for anything you write.

Headline Analyzer by Advanced Marketing Institute (free) – tests the Emotional Marketing Value of up to 20 words.

Email Subject Line Analyzer by Attrock (Free, but will require your email address) – provides an overall evaluation for what they call real-time click potential.

Spam Check tool right here in Hoppy Copy (free) – Gives an overall spam score that lets you know if your email will likely trigger the spam filters and why. It’s super easy and built-in, so you can make changes right there where you’re writing your email. A video in the Learn section shows you how to use it.

Trends and best practices

Most marketers use the 4 U’s - Unique, Useful, Ultra-specific, and Urgency. This is another headline rule that works just as well for subject lines. You might not get all 4 in yours, but the more you can use, the better your results should be.  
After you write your subject line, check to see if it has the 4 U’s.  If it only has one, see if you can add or change it.  If it has two, it’s ok if the subject line is compelling. Three or Four is great.

  1. Unique,  different from the norm.
  2. Useful for the reader, something they need or want.
  3. Ultra-specific, don’t use vague words or numbers.  For example, 237 works better than over 200.
  4. Urgency, give them a reason to open it now.

What else to keep in mind?

  • You need to get to know YOUR own audience. The people that gravitate to YOUR company aren’t the same as the ones that choose your competitors. Therefore, your industry and each company will have different statistics (ie open rates, CTRs) than others. So compare your statistics to your previous performance, not anyone else’s.
  • You should pay attention to which emails you read yourself. Ask yourself why that one caught your interest. What works on you will work for you. You’ll also be able to pick up on new trends as they happen.
  • At the end of the day. Don’t overthink things too much. You’re going to be writing a ton of emails, so test, test, and test again. The more information you have the easier it will be and the better your results.
  • Lastly, be weary of the mantra ‘Always be Closing’. Nobody wants to be sold. Nor are they willing to actively open an email that looks like you might try. Instead, write like you are talking to someone you care about. Without the hugs and kisses.
Josh @ Hoppy Copy
3x founder, CMO, loves email marketing

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