If you want more people to actually open your emails, your subject line is the first (and sometimes only) thing that matters. You’ve probably heard that A/B testing can boost open rates, but maybe you’re not sure how to do it right, or how to avoid wasting time chasing tiny percentage points. This is for marketers, founders, or anyone using email to reach customers—especially if you’re using Postdrips and want real, data-backed results (not just wishful thinking).
Let’s get into what works, what doesn’t, and how to run A/B tests on subject lines in Postdrips without spending all week on it.
Why A/B Test Subject Lines at All?
Subject lines are make-or-break. If people don’t open your email, nothing else matters. But most “best practices” are vague or outdated—what works for a SaaS tool probably won’t work for a local bakery.
A/B testing lets you stop guessing. Instead of going with your gut (or whatever’s trending on LinkedIn this week), you let your actual audience tell you what works.
But here’s the thing: Not every test is worth your time. If you only send a few emails a month, you probably won’t get enough data for meaningful results. But if you’re sending to a list of a few thousand or more, you can learn a lot—fast.
Step 1: Decide What to Test (and What to Ignore)
Before you even log into Postdrips, get clear on what you want to test. With subject lines, less is more. Don’t split hairs over emoji placement unless you have a massive list.
What actually moves the needle: - Different value props (“Save time” vs. “Save money”) - Tone (“Quick question” vs. “You’re missing out”) - Personalization (“Hey [FirstName]” vs. no name) - Length (short vs. long) - Curiosity vs. clarity
What usually isn’t worth it: - Minor punctuation changes - A/B/C/D/E… (just stick to A vs. B) - Testing subject lines on tiny lists (you’ll just get noise)
Pro tip: Write down your hypothesis first. “I think a question will get more opens than a statement.” That way, you’re not just throwing spaghetti at the wall.
Step 2: Set Up Your A/B Test in Postdrips
Log into Postdrips and head to the campaign or sequence you want to optimize. The platform makes A/B testing pretty painless, but here’s what you actually need to do:
- Pick your email: Open the email in your sequence where you want to test the subject line.
- Find the A/B test option: Look for the “A/B test” or “Add variant” button by the subject line field. (If you don’t see it, make sure you’re editing the right email type; some transactional emails might not allow it.)
- Write your two subject lines: Enter your “A” and “B” versions. Don’t overthink it—just make sure they’re truly different, not just a word or two swapped out.
- Set your split: By default, Postdrips will usually split traffic 50/50 between A and B. Stick with that unless you have a specific reason to do otherwise.
- Save and activate: Double-check for typos (seriously—nothing kills a test like a broken subject line). Then save and activate the test.
Don’t bother: With “multivariate” tests (more than two subject lines) unless you’re sending huge volumes. It just muddies the data.
Step 3: Let It Run (and Don’t Peek Too Early)
This is where most people mess up. You launch your test, check results after a few hours, and crown a winner. But early results are almost always random noise—especially with small lists.
How long should you wait? - Small lists (under 2,000): Wait at least a few full sends, or until you’ve got a few hundred opens per variant. - Larger lists: You can make a call sooner, but still give it at least 24-48 hours.
Resist the urge to call it early just because one version is “winning” after 23 opens. That’s not a significant result. If in doubt, let the test run until the open rates start to settle.
Pro tip: If your audience is global, account for time zones. Sometimes version “B” looks better just because it went to people wide awake and checking email.
Step 4: Look at the Right Metrics
It’s tempting to obsess over tiny differences, but not every bump is meaningful.
What matters: - Open rate: The main metric for subject lines. But beware—Apple’s Mail Privacy Protection and other privacy changes mean open rates are fuzzier than they used to be. - Statistical significance: Don’t get scared by the term. Most A/B tools—including Postdrips—will show you if your results are actually meaningful or just random chance.
What doesn’t matter: - Click rate: For subject line tests, this is usually a bonus, not the main thing. - “Feeling good” about the winner: If the data says your “boring” subject line wins, trust the data.
Real talk: If your “winning” variant only beats the other by 0.2%, it’s probably just noise. Look for solid differences—at least a few percentage points.
Step 5: Pick a Winner and Apply the Results
Once you’ve got enough data, it’s time to act.
- Declare a winner: Postdrips usually makes this easy—just pick the subject line with the higher open rate and enough data to back it up.
- Update your sequence: Set the winning subject line as the default going forward.
- Document what you learned: Keep a running list of what’s worked (and what hasn’t). Patterns emerge over time.
- Plan your next test: Don’t stop now—pick another hypothesis and keep iterating.
Don’t: Get stuck in “test paralysis.” If you’re not seeing big differences, move on to testing other parts of your email (like your sender name or preview text).
Common Pitfalls (and How to Dodge Them)
A/B testing is powerful, but it’s easy to waste time or misread the numbers. Here’s what to watch out for:
- Sample size too small: If you only get a handful of opens per version, you’re just flipping a coin.
- Testing too many things at once: Stick to one clear difference between A and B.
- Declaring victory too early: Wait for enough data. Don’t let excitement trump patience.
- Overfitting: Just because one subject line works for one audience doesn’t mean it’ll work everywhere.
- Chasing tiny wins: If your best result is a 0.3% lift, your time is better spent elsewhere.
What Actually Works (and What Doesn’t)
Works: - Testing bold, clear differences (question vs. statement, direct vs. playful) - Using audience insights (if your crowd hates clickbait, don’t test it) - Reviewing tests over time to spot real trends
Doesn’t work: - Obsessing over “best practices” that don’t fit your brand - Copying what worked for someone else without context - Running endless micro-tests with no clear goal
Keep It Simple, Keep Improving
A/B testing subject lines in Postdrips isn’t magic, but it’s as close as you’ll get to reading your audience’s mind. Don’t get bogged down in perfectionism or hype. Test clear, meaningful differences. Pay attention to the data, not your gut. And remember: one good test is better than ten half-baked ones.
Keep it simple, keep iterating, and let your audience show you what works. That’s how you get more opens—and more results—without the guesswork.