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How to write a good subject line

by Stu Elmes



The language you use to communicate with your customers is important.

Whether you’re dealing with a customer complaint, developing a new advertising campaign, or revamping your company’s website, choosing just the right language to help you broadcast your brand’s message to the world is one of the most crucial brand-building tasks you’ll ever undertake. This simple fact is just as true when you’re writing an email subject line as it is when you’re writing any other element of your brand’s marketing strategy.

Use low-quality, spammy language in your brand’s email subject lines, and you’ll turn off your subscribers and negatively impact your bottom line. Conversely, if you use high quality, brand-compliant language in your brand’s email subject lines, you’ll engage more of your subscribers, build robust email marketing dialogues, and ultimately increase your email marketing revenue.

It really is that simple.

Writing good subject lines can (and very often does) mean the difference between email marketing campaign success and email marketing campaign failure.

Here’s how to write good email subject lines and empower your next email marketing campaign to engage your audience more effectively, drive increased revenue, and maximize the value of the awesome email marketing channel…



Bad subject lines

Short-term thinking and the misguided quest for instant KPI improvement have proven a fertile ground for bad email subject line strategies to take root. As a result, the use of bad subject line language has become an all too common practice in the email marketing game.

While such tactics can indeed lead to a short-term uplift in email open rates, audiences wise up quickly, and their impact over the long haul always proves to be negative. This negative impact manifests itself most acutely in two specific ways:

1. Open rates drop over time: There are only so many times you can trick someone into opening an email, and the damage spammy and misleading email subject lines can do to the precious email marketing relationships you’ve built with your subscribers is rarely worthwhile.

2. Higher unsubscribe rates: In a head-to-head comparison with another subject line-optimization vendor spanning 9 sends and reaching an audience of 1 million subscribers for a major eCommerce website, we found that the spammy and misleading subject line tactics our competitor employed led to 27% more unsubscribes over 9 sends than Phrasee’s AI-optimized, brand-compliant subject lines did. Admittedly, industry-wide absolute unsubscribe numbers are low to begin with, but over time they do tend to add up. And, as every email marketer knows all too well, retaining the subscribers you have is a much better mailing list value proposition than finding new subscribers is.



Good subject lines

Now that we’ve established beyond a reasonable doubt that using bad subject line language is a bad strategy, the next logical question is: what is “good” email subject line language? 

Here are a few general rules that every brand and email marketer should be following as a matter of course: 

– Avoid using misleading language: A marketing email’s subject line should always match what is actually in the email’s body. If your subject line promises 50% discounts, your email had better not contain 25% discounts. In the brand-subscriber relationship, honesty matters.   

– Use your brand voice: Your customers have invited you into their inbox because they like your brand. Respect the terms of that invitation by maintaining the brand voice that got you there. Fill their inboxes with spammy, off-brand subject lines, and not only will they quickly tune your voice out, you may be asked to leave and never get invited back again.  

– Offer value: The adage “if you don’t have anything nice to say, don’t say anything at all” holds true in the email inbox. Successful email marketing offers the subscriber something in return for their attention and engagement. The value your emails offer can take many forms, from enticing discounts to entertaining content to stunning visuals, but it’d better be there. If it isn’t, you’ll lose your subscribers’ attention pretty fast. 

– Be different: Guess what; there is a 99.99999% chance that you aren’t the only brand this subscriber has given access to their inbox. In fact, there is a high probability that your marketing emails will appear sandwiched between those of several other brands. An effective email subject line is your best chance to stand out from the crowd and garner enough attention to get that all important email open. That’s why it’s crucial that you use subject line language that doesn’t look like everyone else’s. Get creative. 

– Diversity matters: After tracking the performance of millions of email subject lines over the past 4 years, we’ve learned one really crucial lesson: using diverse language in your subject lines is important. Really important. So important, in fact, that this particular aspect of email subject line language deserves a bit more discussion…   



Language diversity

Every brand’s audience is different. A strategy that works well with one brand’s email list may well fail completely with another. Not only that, but your subscribers’ tastes, preferences, and behaviors will almost certainly change over time. That’s why split-testing your email subject lines is of such crucial importance.  

Subject line split tests can offer you invaluable insights into what kinds of email subject line language work most effectively on your brand’s unique audience, as well helping you adjust the language you’re using to reflect any changes in how your subscribers are engaging with your emails over time. 

However, for split-testing email subject lines to truly be worthwhile, it is essential that those split tests include a wide variety of different words, phrases, syntactic structures, sentiments, and other linguistic elements (like emojis and punctuation). If you don’t experiment with diverse language in your subject line split tests, you’ll only learn half the story. 

Our studies have demonstrated that there is a positive correlation between the diversity of language within a subject line split test and the open rate uplift that split test generates. This makes intuitive sense: little language tweaks (e.g. “should I use a period or exclamation point?”) can lead to small improvements in uplifts, but it is in the bolder, more significant linguistic variations (e.g. “should I say ‘50% off’, or ‘half price’?”) that split-testing’s real impact and value are found.  

Determining how diverse your email subject line language is, how diverse it should be, and how it stacks up against the language diversity your competitors are using is no simple task. Neither is creating, testing, and optimizing the diverse email subject line language that engages your audience and drives the opens, clicks, and sales brands crave.

Luckily, Phrasee is here to help with that. We’ve developed an artificial intelligence-driven solution that takes the guesswork out of writing effective email subject line copy.

There are hidden linguistic diversity opportunities available for every brand. They are the key to improving email marketing performance, boosting open rates, and generating more email marketing revenue.  

Learning how to write good email subject lines just makes good email marketing sense (and dollars!).