Essentials: Writing a good subject line

A good subject line is essential because it directly affects both your deliverability and open rates.

• Bad subject line content can get your email flagged as spam and blocked, or simply ignored by the recipient.
• Good subject line content will ensure your mail gets through to the recipient and improves your open rates.

In this guide, we give you some practical tips on how to get this critical content right every time. First and foremost, remember that most email clients use keyword filters that check for content that indicates spam. You must therefore avoid using words associated with spam at all costs.

General guidelines for writing subject line content

Typical spam subject lines are sensational and overly inviting. To differentiate your own emails from spam, make your subject lines clear, informative, and unequivocal.

Remember that you can also use the pre-header in your email to provide more information, which makes writing the subject line itself a little easier.

Here are some helpful Dos and Don’ts:

Do

• Make your point early in the subject line. You only have 50-70 characters to work with so get to the point quickly. Note that it is only 40-50 characters if the message is displayed on a mobile device.
• Keep your subject lines as close to 50 characters as you can. If it’s too short, then it’s less likely to make the recipient want to open.
• Use normal sentence capitalization (the first letter of the first word only).
• Spell-check your subject line! If your word processing software says there’s a problem, then there is a good chance that it will be flagged as suspicious when sending the email.
• Engage the recipient with genuine questions, such as: Have you ever seen wild dolphins?
• Make the pre-header work with the subject line, e.g.:
  - Subject line: Have you ever seen wild dolphins?
  - Pre-header: Come visit our dolphin sanctuary. Learn more about these majestic creatures
• Use numbers to make your content engaging and add a sense of urgency, e.g.: First 300 responders will receive a discount code.
• A/B test your subject line to see which approach your recipients engage with the most so that you can tailor your approach to maximize open rates.
• Use personalization in the subject lines. Including a customer’s name or other personal data (location, interests, etc.) increases the engagement rate. Personalized subject lines make the recipient feel unique and fosters loyalty to your brand. But do make sure that you set alternative text for the contacts who do not have the necessary data in the fields you use for personalization.

Do not

• Use spammy phrases, i.e. too many sensationalist words or combinations of them make your legitimate content look like spam.

• Try to disguise spam keywords by using bad grammar, for example:
  - Adding spaces in words, e.g.: c a s h.
  - Repeated words, e.g.: cash cash cash cash.
  - No spaces between words, e.g.: cashcashcashcash.
  - Irregular capitalization, e.g.: no capitals used, All Words Capitalized.
  - Misuse of punctuation, e.g.: no-obligation.
  - Inconsistent tenses, e.g.: You can won a prize!
• Use % off in your subject line, standalone % symbols, or currency symbols.
• Use misleading subject lines that do not relate to the content, for example, a newsletter about swimming using a subject line about racehorses.
• Use subject lines longer than 50 characters, as some email clients will not display more than that. The exact number of characters depends on the characters and encoding used, but subject lines longer than 256 bytes will normally be automatically truncated.

If you follow the above advice, you will help reduce the possibility of your mail being flagged as spam and provide a stable baseline from which to grow your response rates.

Remember – it is all too easy to focus your attention on the content of the email itself, and not give enough care to the subject line. This runs the risk of undoing all your hard work because the content isn’t seen.

Emojis in the subject line

How do emojis affect subject line length?

The length of the subject line is primarily important since different devices only display the first few characters of the subject line (50-70 characters as mentioned above). Emojis are wider than normal characters, so you are advised to test how your subject line appears on different devices when you use many emojis. As a rule of thumb, you should count each as 2 characters.

Do multicharacter emojis affect spam filters?

Some emojis are represented by the computer as multiple UTF-8 characters. These emojis are rarely used, but they could in theory make the subject line look longer than it actually is for some spam filters. It is usually not a problem in practice since subject line length is only one (and minor) factor in spam scoring.

Further reading

The Emarsys blog regularly offers advice on improving email performance, such as this related post on award-winning subject lines.