Email Subject Lines That Readers Like and Spam Filters Ignore

The shortest part of the email is definitely the most important part: the email subject line. After all, if a reader doesn’t open your email, then nothing that you wrote, shared, or offered will matter. It’s going into the trash, unread! Following are ways to write subject lines that readers like–and spam filters ignore.

Email Subject Lines That Will Avoid the Spam Filter

Let’s work backward and talk spam. Spam is unwanted, unsolicited, and sometimes virus-infected emails. Spam is bad, and sending spam-sounding subject lines will mark you as spam, even if you are not. This means your emails will never even make it into your reader’s inbox. 

Spam filters evaluate each email you receive to see if it tests positive for spam. When it comes to email subject lines, spam filters are looking for:

  • Poor spelling
  • Over capitalization
  • Over punctuation (especially exclamation points)
  • Trigger words and phrases

When it comes to spam-triggering words and phrases, there are hundreds upon hundreds. You can find the lists at sites such as those at HubSpot and Active Campaign

But if you want an easy way to remember what is a trigger and what is not, ask yourself:

  • Does this subject line read like a scam?
  • Does this subject line read like a promise?
  • Does this subject line discuss free gifts?

If spam filters could talk, they would tell you, “If you sound too good to be true, we’re keeping you away from good people.”

Some of the more interesting trigger words and phrases (and symbols!) include:

  • $$$
  • #1
  • All New
  • Buy
  • Cheap
  • Clearance
  • For You
  • Freedom
  • Friend
  • Get
  • Hello
  • Miracle
  • Teen
  • This Isn’t Spam
  • Why Pay More?
  • Wife

If you still need help practicing how to write a spam-free email subject line, sites like Mail-Tester are a free online service that tests the “spammyness” of your email.

Subject Lines That Will Excite Your Reader

Let’s talk about some sure-fire winners when it comes to your email subject lines and open rates thanks to research done by Active Campaign. Email open rates are higher when:

  1. Your subject lines are short–like three to five words short–so every word is seen in the email preview
  2. Your subject line uses title capitalization–it’s perceived as coming from an authority (who knows grammar)
  3. You use the preheader (the gray text that is next to the subject line in your in-box) to summarize content
  4. There is a number in the subject line
  5. There are emojis

Do you have to do all of these things in every email subject line? Depends on your needs, of course, but we can say without hesitation to always do #1. How can a subject line work if it can’t be read and understood in one glance, right?

According to HubSpot, business emails should also be:

  1. Personal
  2. Valuable
  3. Urgent

Let’s take a look at some real-life subject lines. Email sender Mailchimp shared a client’s open rate for the following subject lines:

A white background with a bar chart. Title says Email Subject Line Performance. There are 5 example email subject lines and their corresponding open rates

The best-performing subject line with a 72.7% open rate had the following:

  • Three to five words (January flash sale!)
  • Valuable (a sale announcement saving the reader money)
  • Urgent (a flash sale–meaning it will be gone soon)

It also had a couple of things that are not supposed to be good:

  • Sentence capitalization (January flash sale!) vs. title capitalization (January Flash Sale!)
  • Exclamation marks (January flash sale!)

But see, title capitalization is a best practice, not a game-ender. And over-punctuation would be a problem (January flash sale!!!), not normal punctuation.

To Sum Up and Your Homework

As you saw, you were given some best practices, some words (and phrases) to be mindful of, and some good resources. Email subject lines are the key to people reading what’s inside versus scrolling on by. If you send an email, prioritize being a better subject line writer today.

Homework: If you still don’t know if you can write a good subject line, try an AI generator like ChatGPT. Give it a prompt that has a role, a purpose, and a target audience, and don’t forget to mention the tone! One caveat: AI is still a computer and you are a human. Add some personality when you find a subject line you like. 


Stay tuned for more information in our Email 101 series!

Emails 101, Lesson 1: The Easiest Email Service Providers to Get You Started

Emails 101, Lesson 2: One Clear Topic per Newsletter

Emails 101, Lesson 4: Quick Lead Generator Magnets That Add Value to Your Emails

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