Good Words vs. Bad Words in the Subject Line

If you’re planning to search this on Google, there is a chance you will fall into a 3-day rabbit hole, you’ll end up with a few pdfs on your desktop and no real conclusion on what to do.

To me, this stuff is not supposed to be learned. You will have to be a full-time email marketer, but you have a business to run.

When my team and I sat down to plan this lesson, we thought the best idea will be to:

  • Simplify
  • Have the information available (to access it whenever needed)
  • And share the tools to make your life easier

So, here we go…


I’ll start by saying that there are 2 things you should consider when writing a subject line:

  • There are words that work better than others and this can affect your open rate
  • And there are words that might trigger a spam filter and block your email

And please be clear about one thing: Many people are spammers and they deserve to be blocked, black-listed and all that stuff, but there are many people that get in trouble by an honest mistake.

The goal here then is: Use words that help increase open rates and avoid words that trigger spam filters.

Good words vs. bad words

There are several studies on this topic. Instead of dropping insane amounts of data and charts on you, I decided to select the most solid and reliable.

I believe you can get the best report today is the Subject Line Analysis Report by Adestra.

You can download it here for free: Subject Line Analysis Report.

Adestra report

Okay, that said, there are still some things I want to cover here…


Some people say personalization works and others say it doesn’t.

Personalization can happen in different parts of an email, but right now we are focusing on its use in the subject line to see if it can help or not.

Retention Science studied 267 million emails and discover that subject lines using the first name of the recipient got 2.6% more opens than those without any personalization.

Personalization in the subject line

“Re:” and “Fw:”

One of the tricks that can get you some of the highest open rates is the use of “Re:” or “Fw:”

What was the other thing I wanted to mention about this… oh yeah: they can also get you the highest percentage of upset people and unsubscribes.

Some marketers use this trick to give the impression of an existing conversation, so people click to realize it’s not the case.

You should never do this. This is the purest form of misleading people.

Re: and Fw:

Spam Filter Triggers

There are words that you must definitely stay away from because they very likely to trigger a spam filter.

I turned it into a Google Doc, you can grab it here: Spam Filter Triggers.


These are some of the tools to help us write a better subject line.

Email Subject Line Keyword Checker

Here is the link: Email Subject Line Keyword Checker

Subject Line Gold

Here is the link: Subject Line Gold

Spam Score

Some ESPs, like Aweber, show a “Spam Score” in the campaigns that are yet to be sent, in case you want to make adjustments.

Spam Score

And it also gives you a quick snapshot of the analysis. In this case, the email is getting a spam score of 1.6 (it’s safely on the green side).

Any score below 5 is okay to send.

Spam Score Analyzer

Check if your ESP includes something like this.


Phrasee is a service that helps you create better subject lines.

It starts at $99/month but has a 14-day free trial.



On the next campaign you send out, you have to do 2 things:

  • Optimize your subject line for better open rate
  • Make sure your subject is absolutely clear of anything that can appear to be spam

Here is the process:

  1. Write a few versions of your subject line
  2. Once you have a couple of possible lines, test them using the tools above
  3. Check you subject line for possible spam
  • Denise

    Hi Francisco, is there a tool that allows to check the titles in spanish? Thanks!

    • Sorry Denise, I’m not aware of any tools in Spanish. I’ll let you know if I find one.

      • Denise


  • Just wondering … how do I read the table above? For instance, the unsubscribe rate for word Exclusive is 22 and for Free -1.3. Especially the negative value, what does it mean?

  • Katie Miller

    Francisco, how often do you use the subject line tools discussed in this lesson? In a previous lesson you emphasized the need to A/B test subject lines every time we run a campaign, forever. Do you also use these tools every single time your run a campaign?

    • Here’s what I do:
      1) I write a few options
      2) I run them through the tool to optimize, I usually change a word here and there
      3) Once I decided on the best 2 options, I use them on a 10%-10% A/B Test and the winner is automatically triggered to the remaining 80%