Getting people to open your emails is half the battle in cold outreach. If your subject line doesn’t grab them, the rest of your message might as well not exist. That’s why A/B testing subject lines is one of the quickest wins in email marketing—especially if you’re using a tool like Woodpecker. This guide is for anyone who wants to move past guessing and actually find out what works for their audience.
Why bother with A/B testing subject lines?
Let’s be honest: nobody knows for sure what’ll make someone open an email. You can read all the “best subject lines” advice you want, but your audience is different. A/B testing is the only way to get real answers, fast.
Here’s what you get when you test subject lines:
- Data, not hunches. You see what actually gets opened.
- Higher open rates. Even small improvements matter when you’re sending hundreds or thousands of emails.
- Less spam risk. You can spot subject lines that tank your deliverability.
But A/B testing is only useful if you do it right, read the numbers honestly, and don’t get sucked into overthinking every word.
Step 1: Set up a campaign in Woodpecker
Before you can test anything, you’ll need a campaign ready to go in Woodpecker. If you already know how to get your contacts loaded and a basic campaign drafted, skip ahead. Otherwise, here’s the quick version:
- Import your contact list. Clean it up first—bounce rates kill deliverability.
- Create a new campaign. Give it a clear name so you don’t lose track.
- Write your email body. Don’t stress about the copy yet; focus on the subject line for now.
Pro tip: If you’re sending to a really small list (say, under 100 people), A/B testing might not be worth it. You won’t get enough data to trust the results. Aim for at least a few hundred recipients if you want to see real patterns.
Step 2: Add subject line variants
Here’s where you set up the actual A/B test:
- In the campaign editor, find the subject line field at the top.
- Click the “+” or “Add variant” button (the UI changes from time to time, but this is usually right next to the subject line box).
- Write your first subject line—keep it short and clear.
- Add your second subject line as a variant.
Woodpecker will split your list evenly and send each subject line to half your contacts. You can add more than two subject lines (A/B/C, etc.), but don’t get greedy. Two or maybe three variants is plenty; more than that and you’ll dilute your data.
What should you test?
- Short vs. long subject lines
- Personalization (first names, company names) vs. generic
- Questions vs. statements
- Direct (“Quick question about X”) vs. playful or curiosity-driven (“Idea for [their company]”)
What NOT to bother with:
- Tiny tweaks (“Hi John” vs. “Hello John”)—these rarely move the needle.
- All caps or weird punctuation—it just looks spammy.
- Overused “tricks” like “Re:” or “Fwd:” to fake a thread. Most people see right through it now.
Step 3: Decide how you'll measure success
Open rates are the obvious metric for subject lines. But don’t fall into the trap of chasing opens at any cost. If a subject line gets the most opens but leads to zero replies, it’s just clickbait.
The best approach:
- Track open rates to pick initial winners.
- But also look at reply rates over time. Sometimes a curiosity-driven subject line gets more opens, but a straightforward one gets better responses.
Woodpecker tracks opens and replies automatically. It also shows you deliverability (bounces, etc.)—if one subject line gets way more bounces or lands in spam, that’s a red flag even if open rates look good.
Step 4: Launch the campaign and wait
Once your variants are set, hit send and…wait. Here’s what to keep in mind:
- Patience matters. Don’t call a winner after 10 emails. Give it at least a few days and a good chunk of your list.
- Don’t touch the campaign once it’s running. Changing subject lines mid-test ruins your data.
- Avoid sending at weird times. If one batch goes out at 2 AM and another at 10 AM, time of day could skew your results.
Pro tip: If you’re worried about deliverability, stagger your send times instead of blasting everyone at once. Woodpecker can space out emails automatically.
Step 5: Read the results honestly
Woodpecker will show you the open rates for each subject line. Here’s what to do next:
- Look for real, meaningful differences. If Subject A gets 38% opens and Subject B gets 37%, that’s basically a tie. Don’t overthink it.
- Check reply rates too. A subject line that gets more opens but fewer replies might not be your winner.
- Ignore small sample flukes. If you only sent 30 emails per variant, don’t trust the results.
Here’s what not to do:
- Don’t declare a “winner” after 10 minutes.
- Don’t automatically pick the highest open rate if the subject sounds spammy or misleading.
- Don’t assume what worked this week will work forever. Audiences get tired fast.
Step 6: Use what you learn—but keep it simple
Once you’ve found a subject line that works, make it your new default. But don’t start testing five new ideas at once. Here’s how to keep it sane:
- Iterate slowly. Test one thing at a time. Change too much, and you’ll never know what actually made the difference.
- Keep a spreadsheet. Seriously, write down what you tested and the results. You’ll forget otherwise.
- Test again every few months. What works now may get stale later.
What actually works (and what doesn’t)
Let’s cut through some common nonsense:
- Personalization helps, but only if it’s real. Dropping a first name in isn’t magic. Referencing something specific to the recipient works better (but takes more effort).
- Avoid clickbait. Subject lines that create curiosity but don’t deliver usually get ignored the next time you email.
- Don’t obsess over emojis. They’re not a silver bullet. In most B2B settings, they look unprofessional or get filtered out.
- Keep it short. Most people check email on their phones. If your subject line gets cut off, it won’t get read.
A few pitfalls to watch out for
- Low volume = bad data. If you’re sending to a small list, don’t trust the results. You need volume for A/B testing to work.
- Email service weirdness. Sometimes, open tracking isn’t perfect. Apple Mail Privacy Protection, Gmail image blocking—it’s not bulletproof. Treat open rates as a guide, not gospel.
- Don’t chase tiny improvements forever. If you’re already at a decent open rate (say, 40%+), focus on your email body and follow-ups. That’s where real gains happen.
Wrapping up: Keep it simple, keep moving
A/B testing subject lines in Woodpecker isn’t rocket science, but it does take a little discipline. Stick to big, clear changes. Don’t get bogged down in endless tweaks. Pick a winner, run with it, and test again in a few months. Most importantly, don’t let “perfect” get in the way of “good enough”—just keep sending, learning, and iterating.