If you’re sending cold emails or running outreach campaigns, there’s one thing you can’t ignore: your subject line. You can have the best pitch in the world, but if your subject gets ignored, no one’s reading it. This guide is for anyone who’s tired of guessing what works, and wants a practical way to figure it out using Sendpotion. Here’s how to actually A/B test your subject lines—and what you should stop worrying about.
Why Bother A/B Testing Subject Lines?
You’re probably thinking, “Do I really need to do this?” Here’s the reality: most people change subject lines based on gut feeling or random advice from some blog. That’s fine if you’re only sending a handful of emails, but if your job or business depends on email replies, that’s just leaving money on the table.
A/B testing takes out the guesswork. You’ll see what actually gets opened and what doesn’t. It’s not about chasing some mythical “best” subject line—just about getting a little better each time.
Step 1: Figure Out What You’re Testing (and What Doesn’t Matter)
Before you even log in to Sendpotion, get clear on what you’re trying to learn. A/B testing only works if you test one thing at a time. For subject lines, that means:
- Test one variable: Don’t change the sender name, email copy, and subject line all at once. Just the subject.
- Ignore tiny differences: Swapping “Hey” for “Hi” probably won’t move the needle. Test bigger changes—length, curiosity, directness, personalization, etc.
- Don’t stress about “best practices”: There’s no universal law for subject lines. What works for others might flop for you. That’s the whole point of testing.
Pro tip: If you’re only sending 20 emails, don’t bother with A/B tests. You need enough volume for the results to mean anything (more on that later).
Step 2: Set Up Your Subject Line Variations in Sendpotion
Once you know what you want to test, it’s time to set up the experiment. Sendpotion makes this part pretty painless—no spreadsheets or manual tracking.
- Log in to Sendpotion.
- Start a new campaign (or pick an existing one).
- Add your email content—write your message as usual.
- Find the A/B testing option for subject lines. Usually, there’s a “+ Add variant” or similar button right where you enter the subject.
- Enter your variations. For example:
- Variant A: “Quick question about your hiring plans”
- Variant B: “Are you still looking for help with sales?”
- Double-check everything. The rest of the email should be identical for both variants. Don’t muddy the waters with other changes.
What to ignore: Don’t get bogged down in making six variants. Two is enough. More variants mean smaller sample sizes and more confusion.
Step 3: Choose Your Audience and Send Smart
For A/B testing to matter, you need a decent sample size. Here’s the honest math:
- If you’re sending fewer than 100 emails per variant, your results are basically a coin flip.
- Aim for at least 200-300 recipients per variant if you want to see real patterns. If you don’t have that kind of volume, you can still test—but don’t read too much into tiny differences.
How Sendpotion handles this: The tool splits your list randomly, so each subject line gets a fair shot. You don’t need to do anything special—just select your list and go.
Pro tip: Make sure your list is relevant. If you’re emailing a random batch of leads, even the best subject line won’t fix bad targeting.
Step 4: Monitor Results Without Overthinking
Sendpotion will start collecting data as soon as emails go out. Here’s what to look for:
- Open rate: This is what your subject line affects the most.
- Reply rate: It’s tempting to chase opens, but if nobody replies, does it matter?
- Clicks (if you include links): Not always relevant for pure outreach, but good to know.
What works: - Focus on open rates to judge subject lines, but keep reply rates in mind. Sometimes a “curiosity” subject gets opens, but a more direct one gets replies.
What doesn’t: - Don’t panic over a 2% difference. Wait until you’ve sent enough emails—results often bounce around in the early days. - Ignore “vanity metrics” like how fast someone opens. It doesn’t mean much.
Caution: Open rates are less reliable these days thanks to Apple Mail Privacy Protection and similar changes. Use them as a directional guide, not gospel.
Step 5: Make a Call and Iterate
After your campaign’s been sent and you’ve got enough data, you’ll see which subject line “won.” Here’s how to act on it:
- Pick the winner: Use the subject line with the better open/reply rate for your next batch.
- Rinse and repeat: Test something new next time—don’t get stuck optimizing the same subject forever.
- Document what you learn: Keep a simple log (even a Google Doc) of what works and what doesn’t for your audience.
What to ignore: Don’t chase perfection. There’s always another thing to test. If you find something that works “well enough,” move on to optimizing your message, targeting, or follow-ups.
Real-World Tips (What Most Guides Won’t Tell You)
- Personalization helps—sometimes: Adding a recipient’s name, company, or specific detail can boost opens, but it’s not magic. If it feels forced, skip it.
- Short often beats long: Most people scan their inbox. Aim for 5–8 words, but don’t force it.
- Questions work, but… Everyone’s doing it. Sometimes a straightforward subject (“Quick intro—John at Acme”) feels more genuine.
- Be wary of “trendy” tactics: Emojis, ALL CAPS, or clickbait can backfire. If you wouldn’t open it, don’t send it.
- Don’t ignore deliverability: If your open rates tank, it could be spam filters, not your subject line. Check your sending reputation and authentication.
Wrapping Up: Keep It Simple, Keep Moving
A/B testing subject lines with Sendpotion isn’t rocket science. Don’t let all the advice out there paralyze you. Pick two solid ideas, set up your test, send enough emails to see a real difference, and move on. Most “optimization” is just about doing the basics well, then repeating what works.
If you’re stuck, remember: the best subject line is the one that gets you more conversations, not just more opens. Test, learn, repeat—and don’t waste your time chasing unicorns.