The open rate of outreach email campaigns depends 80% on the subject line. You can invest in the perfect text, polish the structure, build a logical funnel, but if the subject line doesn’t 'catch' the reader's attention, your email will remain unread. So today, we will talk about how to test subject lines and find the ones that actually work.
Why should we even test the subject line?
Because we don't have a crystal ball. Which subject line will work better — "I have an idea for your website" or "5 mistakes that kill conversions" — we can only guess. Testing will show what actually grabs the recipient's attention.
Subject lines are like news headlines: no one wants to read something boring and obvious. You need to intrigue, evoke emotion, promise value, or just be a person, not a robot.
What can (and should) be tested in the subject line?
For the email not to go directly to spam, and for the recipient to really open it, the subject line should be:
with variables (if you have {{Name}} or {{Company Name}} — use them!);
different for each part of the database (rotations will help to avoid sending 1000 identical subject lines);
without a promotional flavor — no "increase sales by 1600%", leave that for landing pages;
short — ideally up to 50 characters, so it fits in the mobile string.
Here is what is most often tested:
Wording: "Question" vs "I have a couple of thoughts"
Tone: friendly ("Hi, {{Name}}") vs businesslike ("{{Name}}, a question about the website")
Personalization presence: "Idea for {{company}}" vs "I have an idea"
Length: "Partnership" vs "I want to discuss an idea for {{company}}"
Intrigue: "I found a weak spot at {{company}}" vs "About the site — quick"
Presence of links and attachments: sometimes even an empty link to Notion can affect delivery. Check if this influences your open rate.
Email provider: emails from Yandex, VK Workspace, or Gmail may behave differently. Sometimes one provider consistently delivers emails, while another sends them nowhere.
Examples of subjects that work:
"Question about {{company}}"
"{{company}} × {{your company}}"
"Partnership with {{company}}"
"I found a couple of ideas for {{company}}"
The subject line of an email is not a landing page headline. It's a hook to make someone click. Everything else is already inside the email.
And most importantly — you need to test not guesses, but hypotheses. That is, assumptions with logic. For example: "If I use a name in the subject, the open rate will increase, because it creates a feeling of a personal email".
But! To understand what exactly worked — change only one variable at a time. For instance, first test only the subject line. Then — only the text of the first email. Afterwards — the sequence. If you send different subjects at the same time, to different bases and with different texts — this is no longer an A/B test, but a mix. You won't understand what actually influenced the result.
How to test subject lines in Coldy
It’s simple:
Load the database and divide it into parts (for example, 2000 addresses → 1000 for each version of the subject).
Create a campaign and set up 2 email sequences — with different subjects, but the same body text.
Launch the campaign.
After 1–2 days, analyze the metrics: Open rate, Reply rate.
In Coldy, you can immediately see how each sequence performs. And if you are testing more than two options — create more sequences and compare.
What is considered a "successful" subject line?
Open rates are a varied thing. They depend on:
database quality
mailing infrastructure
day and time of sending
market (in B2B one open rate, in B2C — another)
But on average:
Metric | Good | Excellent |
Open Rate | >40% | >70% |
Reply Rate | >2% | >5% |
Important: a high open rate does not always equal success. Sometimes a subject line is clickable, but irrelevant recipients are just being curious. Look at the combination: subject → opening → reply.
5 subjects that showed excellent results among our users
From the experiences of Coldy mailings, here are the subjects that consistently achieve a high open rate:
A question for {{Name}}
I saw {{something}} on your website
A small idea for your team
{{Company}} × {{Your company}}
These subjects are short, clear, and evoke interest. And most importantly — they don’t look like a market banner: no caps, exclamations, or words like "Unique Offer!".
Tips for A/B testing subjects
Test only one variable at a time (the subject line). Do not change the body text, CTA, etc.
Divide the database randomly. The more evenly distributed, the more reliable the result.
Do not draw conclusions from 10–20 emails. A minimum of 100 for each subject, preferably 500+.
Do not "stretch" the weak variant. Even if the open rates are almost the same — choose the winner and continue testing.
Size matters. Do not draw conclusions based on 50 emails. To come close to adequacy — you need at least 500 sends for each subject. Only then will a more or less reliable picture emerge.
Make conclusions and record insights. What worked? Why? How can this be used in future campaigns?
Subject ideas if you don’t know where to start
Here are templates that can be customized for your product:
"{{Name}}, a small proposal regarding {{Company}}"
"Question about [client's topic]"
"How we helped [similar client] increase [result]"
"I found a couple of ideas on your website"
"Can I help with [problem] at {{Company}}?"
And don’t be afraid to test non-standard things. Sometimes a subject made of one word can outperform a long headline showcasing all your expertise.
Conclusion
A good subject line is 80% of the success of your mailing. Don’t write "straightforwardly," don't guess. Test. Even a simple replacement of one word in the subject can give a 15% increase in open rate, which means more leads and sales.
Make the subject your main hook. And remember: Coldy makes A/B testing fast and easy — try it, measure it, find the best wordings, and scale the results.
Want to test your subject right now?
Create a campaign in Coldy and launch your first A/B test — it will take less than 10 minutes.
