[Reply Research] These Words Will Make or Break Your Cold Email Subject Line

[Reply Research] These Words Will Make or Break Your Cold Email Subject Line


If you enjoy decoding the secrets of effective sales engagement as much as I do, you will be happy to hear that we’ve prepared the new issue of Reply Research, a series of posts featuring data-backed insights from the millions of emails sent through Reply.

In the previous post, we already shared tips on how to write better subject lines, based on 4 key parameters: length, letter case, variables, and emojis.

This time, we’ll be focusing on how certain words and phrases used in the subject line influence open and reply rates. So, let’s get started!

Cold Email Subject Lines Cheat Sheet (+70 Top-Performing Templates)

Download the cheat sheet with the words guaranteed to boost your email subject line performance (and the ones to avoid) + 70 ready-to-use templates.

Do numbers in the subject line really help?

The first thing I was really curious to test was the impact numbers might have on your subject line performance. These numbers may be related to the value proposition (e.g., 2X your sales) or simply aim to catch the attention (e.g., scale your 2022 sales or #1 sales engagement tool).

Here are the open and reply rates for subject lines containing numbers compared to the average results.

subject line numbers

As you can see there’s no real impact on the subject line performance. So it’s safe to say that you can use the numbers if you see fit, but don’t expect this to boost your cold email performance.

Often used with the numbers in subject line, the percent sign (%), on the other hand, can dramatically decrease your rates.

subject line percentage

This was a real surprise to me since I always believed that being specific in what you’re offering could really help. It seems that promising to increase your sales by 30% in your subject lines might be perceived as spam or even a scam. So be careful.

Key takeaway: You can use numbers in your cold emails subject lines at your own discretion, but avoid including the percent sign along with them.

Should you use marketing speak in your subject lines?

There are several types of marketing words (also generally considered to be the spam trigger words) that are often used in cold email subject lines as clickbait. Now, we’re not here to discuss if it’s ok or not – it’s a judgment-free zone! 

What we want to figure out is if this approach really works. Could using spam words in subject lines help you drive more opens or replies? 

Well, there’s no simple answer to this question. According to our data, there are three groups of such words, based on their impact on email performance:

marketing words in subject line

  1. Words like click or get tend to harm both open and reply rates. The damage could range from just 7.5% to the shocking 61.5%!
  2. Words offer and free that make the second group of the same category might slightly increase your open rates (after all, who wouldn’t want something free?) while also decreasing the reply rate by up to 7.2%. It seems that such emails rarely deliver on a promise made in the subject line.
  3. What you should pay attention to is the third group of words categorized as marketing speak. This includes words like buy, discount, opportunity, cooperation. When used in the subject lines, they perform surprisingly well driving up to 30.5% more opens and 41.7% more replies.

As you can see, not all spam words are equally bad for your cold email. Use them with caution and make sure to always match the subject line with the value proposition in the message.

Effective subject lines to steal:

  • {{Your Company}} | Partnership Opportunity
  • Cooperation with {{Your Company}}?
  • {{FirstName}}, open for cooperation?

How do ROI-related words impact cold email subject lines?

If people are interested in cold emails promising a discount or opportunity, then subject lines that contain words related to other business benefits should be as successful, right?

These are the words we’ve decided to analyze in our research next. Words like improve, increase, scale and others seemed really promising. Yet, our data scientists surprised me again!

roi language subject lines

All in all, using ROI language in subject lines can significantly damage your open and reply rates, decreasing them by up to 34% and 66.5% respectively. I see two possible reasons for that: Either people are skeptical about any direct benefits offered in a cold email or subject lines like those are overused to the point that they lost their impact.

The words grow and boost seem to be the only exceptions, driving results that are slightly above the average. I would highly recommend running a few A/B tests with them to see if you can get better results.

Bottom line, don’t get carried away with ROI language – such words can significantly harm your open and reply rates (with just a few exceptions).

Effective subject lines to steal:

  • Let's Grow Together
  • {{FirstName}} – {{Pain Point}}
  • Low-cost {{Pain Point}} for {{Company}}

Bonus: Quick question

As an SDR leader, I also receive a ton of emails from other salespeople. And I truly enjoy going through my inbox to see what new trends or creative tactics other teams are using. However, at one point, I couldn’t help but notice that my inbox looked like this.

inbox question

It was overflowing with emails containing different variations of the quick question subject line formula! So I couldn’t wait to see if it was really that good, according to the data.

Drumroll, please!

question subject line

As you can see, it’s not that straightforward. Obviously, the subject lines containing words (quick) question aren’t as effective as many salespeople seem to think. Some recipients might be more open to having a chat with you, but be sure to impress them with your email as well.

On the other hand, mentioning a referral or request in your subject line might help you get more opens and, what’s more important, significantly more replies (by up to 55.9%)!

The key takeaways here are: 

  1. Don’t blindly follow the trends. What works for some people might not be as effective for others, especially if everyone is using the same approach. 
  2. A referral-based cold outreach is always a great idea!

Effective subject lines to steal:

  • Quick Chat About {{Company}}
  • Call between {{Company}} and {{Your Company}}
  • {{FirstName}}, a quick favor?

Cold Email Subject Lines Cheat Sheet (+70 Top-Performing Templates)

Download the cheat sheet with the words guaranteed to boost your email subject line performance (and the ones to avoid) + 70 ready-to-use templates.

Wrapping up

This research aggregated data on millions of emails sent via Reply, so it contains emails to prospects from different industries, countries, job titles, etc. The benchmarks we set for this research were 31.69% open rate and 3.02% reply rate.

If you already have better results with your campaigns (including the ones using the words we don’t recommend), maybe there’s no need to change anything in your outreach strategy. But hey, there’s always room for improvement. Why not experiment and A/B-test different options to see if you can get even better results?

Hopefully, our insights will help you find a few ideas worth A/B testing or even get a few extra replies!

Let us know what your experience is with the mentioned words in the comments. And subscribe for our newsletter to get more of the data-backed insights right into your inbox!

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