Inbox warming takes time, but understanding what happens along the way can make the process much easier to plan for.
What is inbox warming?
Inbox warming is the process of gradually increasing email sending volume after moving to a new platform. It helps mailbox providers recognize your normal sending patterns, build trust in your new sending infrastructure, and continue delivering your emails to the inbox.
The goal isn't simply to send fewer emails during the transition. It's to establish a healthy sending reputation from day one.
Consistent volume and positive engagement help show mailbox providers that recipients want to hear from you—and that your messages belong in the inbox.
That's why email onboarding often takes longer than onboarding another channel, such as SMS. The extra time is intentional. It helps protect your sender reputation while creating a stronger foundation for future email performance.
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Here are answers to 10 of the most common questions marketers ask about the email inbox warming process.
1. Can I do anything before migration to make inbox warming smoother?
Two preparations can make a meaningful difference.
First, gather the most recent open date for each subscriber you plan to migrate. If that information is incomplete or outdated, consider running a final winback campaign before the move. Recent engagement helps determine which subscribers should be included first during warming, since people who have engaged recently are more likely to open an email sent through your new infrastructure.
Second, decide which evergreen campaigns you can use during the warm-up period and prepare that content in advance. Having approved, reliable creative ready to go helps your team follow the warming plan without creating unnecessary delays.
2. Which subscriber segments should I use during warming?
During warming, recency of engagement is usually more important than categories such as VIP status, location, or purchase frequency.
Include each subscriber’s most recent open date when uploading your list, then use that information to build segments based on recent engagement. Other attributes may still be useful, but your implementation contact can help determine whether they belong in the warm-up strategy.
After moving to Attentive, Blenders Eyewear created segments based on engagement during the previous 30, 60, and 90 days. This helped the brand prioritize subscribers who had recently interacted with its emails, giving mailbox providers stronger engagement signals during the transition.
3. What is the right list size to start with?
The goal is not necessarily to warm your entire subscriber list. It is to gradually reach the daily sending volume you expect to maintain once the program is operating normally.
Start by identifying the number of recipients you typically send to each day. At Attentive, we generally recommend warming with subscribers who have opened an email within the past six months.
That audience may represent 10% of one brand’s total list and 80% of another’s. The difference depends on list quality and recent engagement. Once mailbox providers have seen stable volume and healthy engagement, less recently engaged subscribers can be introduced gradually.
4. Should I run A/B tests during the warm-up period?
We recommend waiting until warming is complete.
A controlled approach makes it easier to understand how your new sending infrastructure is performing. If the infrastructure, audience, subject line, creative, and offer are all changing at once, it becomes harder to identify what is affecting the results.
During warming, let the new infrastructure be the primary variable. Once sending has stabilized, your team can resume testing other elements of the program.
5. How can I tell whether bots are affecting my opens or clicks?
Some opens are generated when an email service preloads a message to check or display its content. These are often called machine opens. They can only happen after the message has been delivered, so they still provide some information about inbox placement.
That is why we look at total unique open rates and broader trends rather than relying on a single metric in isolation. If both human and machine opens begin declining, it may indicate that more messages are being filtered into spam.
Automated clicks are more common with addresses ending in domains such as .gov or .edu, where advanced security systems may open and click links while scanning a message. The goal is still to earn engagement from real recipients, but automated activity can also indicate that the email reached the inbox and passed through the recipient’s security checks.
6. Which matters more: the audience or the subject line?
Start with the audience.
A strong subject line cannot compensate for sending to people who are unlikely to engage. Repeatedly emailing disengaged subscribers may signal to mailbox providers that your messages are not relevant, which can eventually affect whether future emails reach the inbox.
Once you have identified the right audience, use the subject line and content to give those recipients a relevant reason to engage.
7. Which performance changes are normal, and which are warning signs?
Because warming begins with recently engaged subscribers, some metrics may initially look stronger than they did on your previous platform.
It is also important to give campaigns time to accumulate results. Depending on how frequently recipients check their inboxes, opens and clicks may continue to populate for up to 72 hours. Reviewing performance on a rolling basis provides a more useful picture than reacting to early results from an individual send.
As long as the rolling averages remain healthy, the warm-up plan can usually continue. A sudden increase in bounces, blocks, spam complaints, or unsubscribes deserves closer attention and may be a reason to pause before increasing volume further.
8. Should I run a winback campaign before migration?
A carefully targeted winback campaign can help update your engagement data before the move. The goal is to identify subscribers who still have a realistic chance of opening, clicking, or purchasing—not to send one final campaign to every address you have collected.
Focus on audiences with a history of measurable engagement or conversion. If you send too aggressively before migrating, particularly when you plan to keep using the same sending domain, that activity may affect the warm-up timeline on the new platform.
9. What should I do if subscribers say my emails are landing in spam?
An isolated report does not necessarily mean your entire program has a deliverability problem.
Mailbox providers continually adjust and test their filtering systems, and placement can vary from one recipient to another. If someone finds your message in spam and moves it back to the inbox, that action gives the mailbox provider a useful signal about the recipient’s preferences.
Look at the broader performance of the program before drawing a conclusion. If unique human open rates remain stable and you are not seeing wider shifts in bounces, blocks, or complaints, the issue may be limited to a small number of recipients.
10. What is the most important thing to remember about email deliverability?
Email strategy cannot stay unchanged indefinitely.
Subscriber behavior changes, mailbox-provider requirements evolve, and audiences become less engaged over time. Regularly reviewing performance and adjusting your approach helps prevent small issues from becoming much larger ones.
The audience should remain the starting point. Email works best when it is useful, relevant, and sent to people who want to receive it. It can be a creative channel, but it is not the right place to try every idea with every subscriber.
An SMS campaign that receives little engagement may underperform, but it does not typically prevent you from reaching that subscriber again. With email, consistently low engagement or negative signals can affect whether future messages reach the inbox—including messages intended for your most loyal customers.
Protecting deliverability is therefore about more than one campaign’s results. It is about protecting your ability to continue reaching the people who want to hear from you.
