5 Best and Worst Words for Email Subject Lines

Lisa Cook on Email Marketing

Oh, subject lines. So difficult to write, yet so important to the success of any email campaign.

Getting your email noticed in a crowded inbox can be a challenge, but an effective subject line goes a long way towards increasing open rates on your emails.

(And you can’t get a conversion or even a click if nobody opens your email in the first place.)

Writing that email subject line can be a challenge, and there are a lot of tips out there from various sources. The problem is that most of this advice is given in a vacuum; it doesn’t take into account the sender’s industry or the type of email campaign being used.

But when paired with the knowledge that you have about your audience and your industry (and a generous dose of testing and analysis), subject lines can be improved and become more effective.

Factors That Influence Subject Line Construction

We’re going to be taking a look at what recent research indicates are the best and worst words to use in your email subject lines, but keep in mind that each one may not be the best or worst for your particular needs.

The best and worst subject line content will definitely be affected by the type of business you are in, the type of email you are sending, and what you want to accomplish with the email.

Your focus will be different if you are a B2B organization than if you are a nonprofit organization, as well as if you are sending a monthly transactional email recognizing a payment versus sending an appeal to donors after a catastrophic event.

From a general perspective however, there are some words that are good to use and some others that you should avoid. Following are five of the best words for use in email subject lines and five words that are the worst.

5 Best Words for Email Subject Lines

Some of the most detailed investigation of email subject lines comes from Adestra, who followed up on their much-read 2013 study with a new version in 2015. Their Subject Line Analysis Report looked at over 125,000 individual email campaigns, 3 billion attempted email sends, 400 million opens, and 55 million clicks.

According to their data (and a few other studies that we looked at) you’ll rarely go wrong with these five words:

“Thank you” (or “Thanks”)

A sincere, “thank you” is always appreciated, but make sure it’s relevant to your message. More often than not this phrase belongs in transactional emails.

“You” or “Your”

Personalization can be a very effective way to build a relationship or enhance on one that is already in place. But keep in mind that if you are going further and using actual subscriber names in your subject line you need to make sure your data is clean enough to do it well.


The stars here represent different topics; separating them with the pipe symbol allow you to communicate a bit more in your subject line without wasting characters on transitional words. For example, “Marketing webinars | Win a free trip | Limited space | Register Today.”


This can indicate an existing relationship, so emails using this term in the subject line may be opened more often. As with, “Thank you,” this word typically belongs in transactional emails. Don’t shoehorn it into every single subject line for the sake of more opens.


Monthly can also indicate an existing relationship, which may cause emails to be opened more often. It also indicates that you don’t email very often, a fact that overwhelmed subscribers probably appreciate.

5 Words to Avoid in Your Subject

Many of these terms are common sense ways to avoid getting flagged as spam, but when writing subject lines you also need to consider why your subscribers signed up with you in the first place.

Except in very rare industry-specific cases, people aren’t going to be interested in getting journals, reports, or whitepapers via email. How are those going to compete with new food recipes, pictures of cats, or urgent professional emails?


No list of no-no’s in subject lines would be complete without this term. Using “free” in an email subject line is a very good way to hurt your sender reputation. Try to express the value of what you’re offering in different ways.


Using the dollar sign, especially in multiples, can also cause the email to be seen as spam. Consider an emoji featuring dollar bill or dollar signs instead.


Often associated with scam money-making plans, this term is another quick way to be seen as spam.


Recipients are naturally skeptical of emails whose subject lines use this word, as not much can actually be guaranteed. Remember you’re dealing with jaded readers who are looking for any excuse to cull your message from their inbox.

“Whitepaper,” “Journal,” “Report”

Sales and marketing folks may argue that whitepapers are often offered to provide value to recipients, but those recipients need to be uniquely receptive to this content to make it effective. Many times when people are in need of them they search them out online themselves. If you must send whitepapers via email, try using your subject line to talk about, well, the subject of the whitepaper instead.

Other Subject Line Considerations

There are other words or punctuation that can help or hurt open rates depending on how you use them. For instance, an offer of a percentage off can help, but the larger the percentage is the more likely it is that the offer can be seen as not being genuine.

An offer of 10% off will likely get a better open rate than on of 50% off. Keep in mind that if you offer discounts all the time, likely recipients will just tune out.

Exclamation points in a subject line can be tricky as well. Use one, and it can help convey a sense of excitement and/or urgency. Use three, and you can find your email ending up the spam folder. While spam filters look at much more than just trigger words, avoiding them will only help.

The content of your subject line needs to convey the value of opening the email immediately to the respondent and you should strive to be clear rather than catchy. This is more important now than ever, when many of us are reaching out to international audiences.

Since the subject line’s job is to get the email opened, make sure once the email is opened the email content fulfills the promise made in the subject line. There’s no quicker way to build mistrust of your emails than to fail on that promise.

Analyzing Your Own Subject Line Performance

If you have good records of the emails that have been sent out over time, spend some time compiling those subject lines and the open and click rates of the emails.

Then take a look and identify the words that have been associated with the best performers for each of your types of emails.

And finally, move on to A/B testing with your new emails and using that body of knowledge as you go forward.

No Magic Subject Line Bullets

While there is no magic way to absolutely know what subject lines will work in every instance, avoiding words that are indicators of spam and testing and analyzing opens and click rates, and then applying what you learn will improve results over time.