How to Boost Your Cold Email Open Rate by 16.4%

What’s the difference between campaigns with a ~40% and a 60% open rate?

The subject line.

There are a lot of opinions about what makes or breaks a subject line, but lately a lot of specialists on LinkedIn have opined that a good subject line is the following:

• 3-4 words long
• All lowercase
• Personalized, unique, and driven by the value prop (i.e. not “one size fits all”)

I try to avoid taking peoples’ opinions at face value, so I decided to run my own A/B experiment and see what results I got.

TLDR:

• The goal: Learn which type of subject line gets the highest open rate
• The tactic: Run an A/B test for subject lines
• The result: Grow campaign open rates by ~16% using the new subject line structure

Evolution of subject lines

The previous rule of thumb for a good subject line was that it had to be:

• 4-6 words
• Sentence case
• Personalized

Chances are your personalization was limited to placeholders for the person’s name and – if space permitted – their company. As a result, subject lines would look something like this:

{First name}, want to level up {Company}’s outreach?

If you filled in the placeholders, you’d see:

Yuriy, want to level up AiSDR’s outreach?

This was enough to minimize the risk of Google or Microsoft marking your outreach as spam.

The problem is that people aren’t opening these emails anymore. Throw in the fact that email service providers are making their spam policies stricter, and the subject lines of old just no longer cut it.

Now, most sales specialists recommend that subject lines contain 3-4 words, use lowercase for all words, and emphasize the unique value proposition in a personalized way.

But to be absolutely sure, I ran my own experiment.

Here’s what I did.

Step 1: Set up the campaigns

I set up two campaigns in AiSDR with the same:

• Number of leads that passed bounce checks (I added 340 leads to each campaign)
• Ideal customer profile
• Sales persona
• 7 mailboxes that are warmed up and rotating
• Sending algorithm
• General email infrastructure
• Subject lines configuration:
  • AI-generated
  • Unique
  • Personalized
  • Value-driven

The only differences were the parameters for the subject line, which were:

• Campaign A: Subject lines are sentence case and limited to 4-6 words
• Campaign B: Subject lines are lowercase and limited to 3-4 words

Step 2: Run the campaigns

This part is fairly self-explanatory.

Once the campaigns were set up, I launched them.

Since I had the same mailboxes running two campaigns, it took 2-3 weeks to engage all leads and progress from first-touch emails to follow-ups. This is mainly because AiSDR limits outgoing emails to 50 per mailbox in order to avoid overheating and triggering spam filters.

The main action you need to take here is make sure the campaigns are running, performance stats are being tracked, and that the mailboxes stay healthy.

Step 3: Review the campaign results

After all leads have been engaged at least once, you should have enough data to analyze it.

My experiment yielded these results:

• Campaign A: 43.4% open rate
• Campaign B: 59.8% open rate

The new subject line paradigm significantly outperformed the old way.

The Result

While it’s possible that Campaign B’s leads were just generally more willing to open cold emails, it’s impossible to filter leads based on their inquisitive nature.

Other approaches like intent-based and signal-based outreach would likely further improve the chances of opens, as well as the likelihood of getting a positive response.

It will take a few more trials to validate the original experiment’s results, but at the moment, we can safely say that for best results, subject lines should be:

• 3-4 words
• Sentence case
• Personalized for name and value