Judging The Book By Its Cover - Email Subject Line Best Practices For Increasing Open Rates

Digital Marketing

You may have engaging imagery and highly-personalized content or offers, but all your hard work will be for nought if you don’t have carefully constructed email subject lines that compel customers to open in the first place.

Research reveals that 35% of customers open an email solely because of the subject line text, while a whopping 69% use this text to determine which emails to report as spam.

This post outlines some best practices for creating subject lines that will increase your open rates, and get your recipients engaging with the content inside that you worked so hard on.

Pause to Reflect

A great first step when writing subject lines is to put yourself in the shoes of the email recipient.

Emails are such a big part of our lives that a little reflection on how you personally have responded to different subject lines in the past - what you found intriguing and what you found irritating - will provide a great test to apply to those you create.

Over time you will develop a deep understanding of your various target demographics by testing keywords and experimenting with segmentation to  build detailed customer profiles that can help guide your email marketing initiatives.

Test Early, Test Often

While all of the information provided in this post will help you towards implementing best practices for your email subject lines, it’s impossible to say with certainty what works best for your brand, and your unique list of recipients.

Conducting A/B testing represents industry best practice for assessing the effects of changes to the keywords used or the types of CTA you’re employing within subject lines upon your open rates. Mailchimp provides a simple,  user friendly way to setup and manage A/B testing.

Be Concise

Create concise email subject lines.

In marketing communications there is always a tension between catchiness and clarity. Many people may remember the Puppy-monkey-baby Super Bowl 2016 commercial, far fewer will likely associate this ad with it’s creator, Mountain Dew. But then again half the fun of Super Bowl commercials is trying to ‘figure them out’.

In the case of subject lines, your recipients aren’t looking for hidden meaning, they aren’t searching for pop cultural references, and they definitely aren’t looking to be surprised.

Ambiguity and uncertainty are stepping stones towards concern and mistrust. Keeping your and clear email subject lines have shown have open rates 541% higher than those that aren’t.

This isn’t to say that you can’t have a little fun with your subject lines.

Compelling Calls to Action

Better Hurry, This Blog Content Will Only Be Available for Download for the Next 24 Hours.

That’s not true, but creating a sense of urgency has proven itself time and again to be a powerful tool for increasing open rates and user engagement.

This can be as simple as planting the idea of a missed opportunity within your recipient’s mind (“5 Tips for Doubling Your Summer Sales - If You Start Now “) or can be applied to remarketing campaigns with the offering of limited time discounts (“Buy One Pair and We’ll Give You Two More - Today Only”).

However, (and we’ll say this again for emphasis below) never make promises in your subject line that your email content can’t deliver on.

Nobody likes clickbait.

Examples of These Ideas in Practice

Funny, chatty:


Urgency, enticing: Untitled1

Relevant, timely:


Practice Personalization

Personalizing email content has proven to dramatically improve recipient attitudes toward emails, and even increase conversion rates (see our post on abandoned cart email strategies).

This can be as simple as addressing recipients by their first name, while Industry giant, MailChimp has research that suggests including a relevant city or location name converts even better.

If you’re actively segmenting your email lists into relevant demographic groups of subscribers (by location, gender, age etc.) you already have the means to personalise your subject line content and increase your open rates.

If you aren’t, you’re missing out. Personalised subject lines are 22.2% more likely to be opened.

Count Characters

The industry standard is to keep your subject line to 50 characters or less. That said, it’s important to remember how your subject lines will be displayed to your recipients.

For example, 40% of emails are opened first on mobile devices, where screen size restrictions mean that only the first 4-7 words are shown.

Even if you create a concise 50 character subject line that displays well on desktops, your message may be incomplete or incoherent to almost half your recipients.

For this reason it’s essential that all subject lines waste no time (or characters) in communicating the value their content represents to your customers.


Emojis are a great way to communicate a lot with only a few characters. With the exception of Outlook 2003, emoticons are supported across all major email platforms and operating systems and haven’t shown to flag spam filters.

That said, it’s important that you use them sparingly to ensure that they never reduce the clarity of your subject line message.  Different devices are known to substitute their own icons  and there is always the risk that unsupported emojis will display as “☐”.

For best results:

  • Limit your usage to the more common emoticons
  • Place emojis at the beginning or end of a subject line
  • Never let your subject line message depend on the emoji eg:  “[shoe emoji] clearance”
  • Always run tests across different devices and email platforms before sendout

Don’t Get Marked as Spam

Don’t solicit recipients with lines like “Open Me!”

Don’t make promises in your subject line that your email content can’t deliver on.

Don’t use impersonal, one word subject lines eg: “Hi!”.

Don’t use subject line generators. They might save time, but recipients will appreciate the personal touch.

Spam Filter Target Keywords

As email marketers, the Spam Filter is like an aggressive personal trainer at the gym or a professor with impossibly high marking standards. In the heat of the moment they may come across as an obstacle, but ultimately they push us to work harder to achieve our goals and, most importantly,  ensure that creating a great customer experience is always top priority.

Spam Filters are constantly being updated to flag new keywords (eg: “$$$” and “Earn $”and send email content straight to the trash without ever being seen by the intended recipient, so it’s important to stay up to date. Mailup has an extensive list of flagged words available here.

Subject Line Etiquette


Never add unwarranted email prefixes like “re:” or “fw:” to your subject line. Your recipients won’t appreciate it and it is unlikely to pay off even in the short term, with research identifying that emails with “fw:” in the subject line are opened 17% less than those without.


Punctuation should only be used for message clarity and should never be over used in ways that may come across as spammy to the recipient, eg: “!!!”, “???”.



Aside from the golden rule above, subject line capitalization rules are largely a stylistic decision and should take into account your in-house standards. Two common  examples of capitalization style rules are:

  • Title capitalization, i.e. capitalize the first letter of every word except minor words like ‘the’, ‘and’, ‘of’ etc.
  • Traditional capitalization, i.e. only capitalize the first letter of the  first word of the subject line and any subsequent proper nouns.

Don’t Forget Spellcheck

In subject lines, as in life, typos are not your friend. Draft your emails in a word processor that has a reliable spell check function and always proofread your copy.

Subject Line Graders

Keeping track of all the do’s and don’ts can be quite a task if you’re just starting out.

Fortunately, there are great resources available online including subject line graders where you can paste in your proposed subject line copy and the grader identifies any red flags and provides best practice recommendations.


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