How to write the perfect email subject line: complete guide

Avatar photo Pablo Díaz · 15 Dec, 2022 · Email Marketing · 5 min

Cómo escribir el asunto de email perfecto: guía completa

An email marketing campaign consists of several elements that, as a whole, will determine whether a campaign is successful or not. The subject line of the email is one of the most important and its effectiveness will depend to a great extent on resulting in a successful campaign. By following a few simple best practices, you will be able to get the most out of the emails you send to your subscribers. In this post, we are going to teach you everything you need to know to come up with the perfect subject line for your campaigns, and boost your open rate.

What is an email subject line

The email subject line is the line visible from the subscriber’s inbox, which will indicate what the content of the received email is about. It is one of the three visible elements, along with the pre-header and the sender’s name, that a user will see without opening the email.

The importance of having a good email subject line

As we have said, the subject line is one of the first things the subscriber sees in an inbox likely to be brimming with other emails. So, it is the element that will determine whether or not your subscriber opens the email, which has a significant impact on the open rate, one of the most important metrics in email marketing campaigns. Without leaving aside the rest of the elements of the email (template, content, links, CTAs, etc.), if you do not have an effective subject line, your email will remain unopened.

Good practices

Below is a list of best practices when it comes to writing a good subject line that will boost the efficiency of your emails.


One of the first key points is that, as we have said, the subject line will help the reader to understand what to expect from the email and, based on that, to decide whether to open it or not. It is therefore advisable to come up with a summary so that this sentence reflects the content of the email in a reliable manner, to incite the reader’s interest.

What is the right length.

The answer to this question is: it depends Each mail manager shows a different number of maximum characters in your inbox: for Gmail it is 70, but others show less. To this you must add the range of devices on which subscribers check their email, with their respective differences in screen size. If the subject line is very long, there is a good chance that some of it will be cut off. On the other hand, if it is too short, it will be hard to squeeze in the necessary information to awaken the reader’s curiosity. It is therefore a matter of reaching some sort of middle ground where you are able to express the content of the email in a concise and effective manner.

Use keywords

Include important words related to the theme of the campaign and, if possible, right from the get-go. In this way, you will ensure that they are read on any device and in any situation, and will easily encourage the reader to open the email to learn more. On the other hand, try not to use words that are not needed and take up space such as “Hello” and other types of greetings that you can include in the body of the email. This will also help users who use filters or searches to find emails at a later date.

Awaken curiosity

Although the subject line is used to give the subscriber an idea of what the email is about, you must hold something back to maintain that curiosity that encourages them to open it. It’s not about fooling (as we’ll see later); it’s about trying not to give away too much, making it not worth clicking on your email.

Use the right tone

As is the case with everything related to the content of your campaigns, it is of the utmost importance that you know your subscribers, by adapting the tone when addressing them. Gather as much information as possible, and try to find a voice with which they feel identified and comfortable receiving your emails.

Carry out A/B test

Performing a test for emails with two subject lines, sending them to a small part of the list and seeing which one works better, is a good practice that can significantly boost open rate. Carrying out this type of experiments is very straightforward, and here you have all the information on how to carry out A/B tests in your campaigns.

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What NOT to do in email subject lines

It is just as important to know the best practices that can lead to success as it is to avoid those that can hinder the results of your campaign.

Avoid clickbait

Although we have talked about how the subject line should awake just enough curiosity for your subscribers to open the email, you must avoid falling into the trap of clickbait. If you put a misleading subject line, your email may be opened the first time, but if the content does not deliver what you offered, it will lead to frustration and result in that subscriber not opening successive campaigns or, even worse, unsubscribing or marking your mailings as spam.

Forbidden words

You should also bear in mind that there are certain words that set off the spam filters’ alarms, since the emails that contain them are usually deemed to be spam or contain fraudulent content. These are mostly terms related to money or financial institutions, medications, prizes, sexual content, etc.

Types of subject lines and examples for your campaigns

Below are some examples of campaigns and subject lines that can boost your open rate.


The idea is that, at a glance, the subscriber knows the relevant information about the content. “New shop opening near you”

Create a sense of urgency

Creating a sense of urgency is a good way to boost open rates. “Available for the next two days only: download this e-book”


Personalisation always gets good results. “Pablo, we’ve got something that could interest you”

Lists and numbers

Lists have been very popular content in recent years for one reason: it’s hard not to read them. “15 things you need to know to come up with a good email subject line”


Another effective approach is to use question marks to challenge the reader. “Do you know how to get more customers through email marketing?”

“How to”

How-tos are also often very effective in awakening curiosity. “How to build your database efficiently”


In particular in the case of sales of a physical product, announcing the scarcity of the product is synonymous with success. “Air Jordan Classic 82: last units”


Although always being careful, a more confrontational tone can work. “Why your marketing strategy is wrong”


An imperative tone (again, being careful not to go too far) can work well when directly challenging the subscriber to take an action. “Join our community”


The use of statistics and percentages can lead to good results, in particular in didactic and informative contents. “The great reason to use SMS Marketing: 98% open rate”

Sneak peak

A “sneak peak” at devices or other types of content that interest the reader will certainly awaken their curiosity. “A sneak peak of the new iPhone”


Fear Of Missing Out. Playing on this psychological factor also tends to work. “Are you going to miss out on this webinar?”


The email subject line is very important when it comes to the success or failure of your email campaigns. It is worthwhile, therefore, to keep these best practices in mind and take the time to think it through, try it out and measure the data to improve it in successive campaigns. If you have questions about this or any other email marketing-related topic, contact us and we will be more than happy to help you.

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Escrito por Pablo Díaz Follow Linkedin