How single opt-in and double opt-in actually differ
Every email list inside your ESP has a setting controlling how new contacts get added. With single opt-in, someone enters their email into your pop-up or at checkout, and they are immediately an active subscriber on your list. They start receiving your welcome flow right away without any extra verification.
Double opt-in adds a mandatory gatekeeper. When someone submits their details, they receive an automated confirmation email asking them to click a verification link. Until they click that link, they do not appear as a subscribed contact on your list, and they receive zero marketing emails from you.
That extra step creates friction. Every additional hoop you make people jump through reduces the total volume of leads who make it to the other side.
Why Klaviyo sets double opt-in by default
In Klaviyo, every newly created list defaults to double opt-in. The platform does this to protect server reputations out of the box, ensuring that only valid, engaged inboxes receive mail from day one.
To check or change this setting, head to the Lists & Segments tab in Klaviyo. Click on your main subscriber list, open the Settings tab, and you will see the opt-in process setting clearly displayed. From there, you can toggle between single and double opt-in with one click.
For almost every client account we manage across the agency, I switch that setting to single opt-in straight away. Default settings are rarely optimal for growth.
The real cost of keeping double opt-in on
When you keep double opt-in enabled, list growth slows down significantly. In ecommerce, attention spans are tiny. A customer signs up expecting an immediate discount code on their screen or in their inbox, not an administrative task asking them to confirm their subscription.
A substantial percentage of legitimate shoppers simply never open that confirmation email. Some miss it because it lands in promotions, while others simply lose interest after 30 seconds. Those are genuine prospective buyers who never receive your welcome flow or your weekly campaign drops.
Having driven roughly $5.8 million in email and SMS revenue for our clients, I prefer capturing the maximum number of leads possible. As long as your account health is clean, leaving double opt-in on leaves money on the table.
When you genuinely need double opt-in
There are specific scenarios where turning double opt-in on is the right call. The primary reason is resolving an existing deliverability issue.
If your emails are regularly landing in spam folders, or your bounce rates are creeping up above acceptable thresholds, you need to tighten the gates. Double opt-in guarantees that fake addresses, mistyped domains, and unmonitored accounts never make it into your active sending pool.
You should also switch to double opt-in if your store gets targeted by list bombing (automated bot attacks that flood signup forms with thousands of scraped email addresses). Bot traffic causes massive bounce spikes and spam complaints, which can tank your sender reputation in 48 hours. Double opt-in stops bot attacks instantly.
How to protect your list without double opt-in
If you want the volume benefits of single opt-in without wrecking your deliverability, monitor your account health proactively. Head to the deliverability hub in Klaviyo. If your performance status is sitting at good or excellent, single opt-in is perfectly safe for your store.
One practical tactic I like to use on signup forms is setting clear expectations on the success screen. Tell the customer directly: "Your discount has been applied, and we have sent a backup copy to your email inbox."
I always recommend displaying the discount code immediately on the form success state while also sending it via email. This creates an immediate open on your welcome email, training inbox providers like Gmail and Apple Mail that your messages are wanted.
Final Thoughts
Double opt-in provides cleaner data at the expense of list scale, while single opt-in maximises subscriber volume and immediate revenue. Unless you are dealing with active deliverability penalties or bot attacks, switch your main list to single opt-in and let your welcome flows do their job.
Fix your email capture and infrastructure
If you are unsure whether your forms and deliverability settings are properly configured, we can audit your setup and optimise your flows. Take a look at our Klaviyo account setup service to get your list growth firing properly.
