Email warm up guide

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What is email warm up and why does it matter?

Inbox providers are inherently cautious. They’re unlikely to trust large volumes of email from domains that are new to sending or have recently changed behavior. Email warm up is the process of gradually increasing your sending volume while maintaining high-quality performance—low spam complaints, minimal bounces, and strong engagement (opens and clicks). A successful warm up shows inbox providers your emails are wanted, expected, and trustworthy. It also gives you a chance to catch deliverability issues early on. Warming up isn’t just about ramping volume—it’s about proving that your sending behavior is healthy and your audience is engaged.

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Do I need to warm up my email sending?

You should warm up your sending if:

• You send over 2,000 emails per day.
• You’re new to OneSignal Email. Our IPs are warm, but sending from new IPs resets trust with inbox providers.
• You’re using a new sending domain or subdomain.
• You’re increasing your daily volume by more than 20%.
• You haven’t sent high volumes in the past 30 days.

Even at lower volumes, consistently maintaining good sender reputation and engagement is essential.

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How long does warm up take?

It depends on your sending goals—how much you ultimately need to send daily. As a rule of thumb:

• Increase daily send volume by ~20%.
• Advanced senders with excellent reputation may ramp up by 30% daily, but only if engagement stays high.

Here are conservative warm up time estimates based on a 20% daily increase, starting at 300 emails:

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How do I warm up my email sending?

Email warm up is all about gradual volume increase and strong engagement.

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Start with your most engaged recipients

Begin with your most active users—those who open, click, and engage with your emails.

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Warm up methods

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1. Implement low-volume campaigns first

Start with smaller, behavior-based sends like:

• Welcome emails
• Rewards or loyalty emails

These are naturally lower in volume and show consistent performance—ideal for early-stage warm up.

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2. Use OneSignal’s email auto warm up feature

OneSignal’s Auto Warm Up automatically distributes a single email over time. Great for non-urgent, delay-tolerant emails such as:

• Newsletters
• Evergreen content
• Onboarding flows

Specify your audience and let OneSignal pace the delivery to avoid volume spikes. Run multiple Auto Warm Up emails back-to-back until you reach your desired daily volume.

How to use Auto Warm Up

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Select Auto Warm Up

1. Compose Your Email: Start creating your email message as usual.
2. Select Auto Warm Up: Within the “Delivery Schedule” section, you’ll find the option to “Send with Auto Warm Up”.

OneSignal will automatically generate and display a sending schedule. This schedule is generated based on your past email delivery volumes, scheduled Auto Warm Up emails, and the size of your current audience.

OneSignal calculates the send schedule based on sending activity of the last 3 weeks. If there is a time gap in sending, the system might assume a lower baseline volume, which affects the recommended warm up schedule.

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Monitoring the Auto Warm Up

After sending, you can monitor the progress of sending by viewing the scheduled and total sent messages in the Email message report.

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Adjusting the start date of your message

To set a different start date (default is to start the next day), ensure “Send with Auto Warm Up” is selected.Then:

1. Click on the Edit Auto Warm Up
2. Select Start Date & Time change the date.

Typically, Auto Warm Up will start the next time it is 9am, which might be a day ahead. You might notice that you can’t change the start time for a recommended schedule, to change the Start Date and Time select Custom Schedule first.

Recipients are selected at random from your audience, and the email sends are distributed throughout the day. Your Audience members will only receive the Email once per Auto Warm Up campaign.

Advanced Auto Warm Up Options

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Customize your warm up schedule

If needed, you may customize the warm up schedule to be more aggressive or conservative.

1. Access the Schedule: Click Edit Auto Warm Up to view all Auto Warm Up Schedules and to customize the schedule.
2. Make Adjustments: You can adjust the number of emails sent each day or the duration of the warm up period.
3. Save Your Custom Schedule: Once you’ve made your changes, save the schedule.

If you customize part of your schedule, OneSignal can auto-fill the remainder based on your adjustments. This ensures a continuous and effective warm-up process. The auto-fill will use the default incremental increase of the sending volume by 20% per day.

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Sending multiple auto warm up emails

When conducting an Auto Warm Up process, reaching your desired daily volume may require sending more than one email. Sending Sequential Auto Warm Up Emails to achieve your target daily email sending volume.This approach ensures a gradual increase in volume, aligning with best practices for auto warm up.

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Scheduling your second email

When you’re ready to send a second Auto Warm Up email, OneSignal automatically schedules it for the next available time slot.

• This usually occurs after the completion of your first email’s auto warm up cycle.
• No need to manually adjust the date for the second email; OneSignal calculates the optimal time for you.

You are more than welcome to move the start date, and customise the schedule of any Auto Warm Up emails. We recommend making sure that the total volume of emails being sent meets the recommended warm up schedule. If you have any questions, always reach out to support@onesignal.com.

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Cancelling an active warm up email

To cancel an active Auto Warm Up email message:

1. Navigate to the Email page of your OneSignal Dashboard.
2. Navigate to the email you want to cancel.
3. Click on the options menu (three vertical dots) on the right side.
4. Select “Cancel”.

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3. Use Split Branching in Journeys

For Journey-based sends, combine:

• Split Branches
• Wait Nodes

This lets you control volume across multi-step flows while maintaining logic and timing.

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Summary

Warm up is essential when:

• You’re new to a platform or domain.
• Your volume is increasing significantly.
• You’ve had sending inactivity.

Start small. Prioritize engagement. Monitor performance. A strong warm up leads to better deliverability, long-term inbox placement, and higher ROI.

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FAQ

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Do I need to warm my subdomain if my domain is already warm?

Yes. Each subdomain is treated uniquely by ISPs, so warm up each one separately even if the main domain has a good reputation.

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Why do I need to send more than one warm up email?

A single warm up email may only reach a fraction of your target volume because delivery is distributed over many days. Multiple sequential emails ensure a gradual, steady increase that builds sender reputation without triggering spam filters.

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What is the difference between domain and IP warm up?

Domain warm up is required whenever you migrate to a new email platform. It involves gradually increasing volume from a new or inactive domain to establish trust with ISPs. OneSignal can automate this process. IP warm up is required when using dedicated IP addresses that haven’t been previously warmed. It focuses on building reputation for the IP address itself. OneSignal automates IP warm up in the background for dedicated IP customers.

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What about OneSignal vs. bringing your own ESP?

With third-party ESPs (e.g., SendGrid, Mailchimp, Mailgun), you manage both IP and domain reputation. With OneSignal Email, we manage your IP reputation, but maintaining a warm domain with a good reputation is your responsibility.

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What should high-volume senders consider?

High-volume senders should typically start a new subdomain when moving to a new platform. Using the same subdomain across different ISPs can lead to DNS issues. Starting fresh is often simpler than transferring an established domain.

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What is the recommended warm up schedule?

Start at 300 emails per day and increase by 20% daily. Here is a sample schedule:

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Why are only some of my emails failing?

This typically indicates the volume is too high for your domain’s current ISP reputation. Look for a 602(too old) error in Audience Activity. Failures may only occur with certain ISPs like Google or Outlook. Pull back to the volume that was previously delivering successfully and slowly increase daily.

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Can I edit an email that is being warmed up?

No. Editing is not available once a warm up send has started. To cancel, navigate to the email index, click the three-dot menu on the row, and select Cancel.

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How do I A/B test an auto warm up email?

Create two separate email messages with different segments and customize the schedule of both. For a 50/50 split, create two segments of equal size, then select auto warm up for each and customize the start date and volume per stage so they sum to the recommended values.

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What is the difference between dedicated and shared IPs?

By default, emails are sent from a shared IP that helps maintain reputation — you only need to warm your domains. A dedicated IP is used exclusively by your account, giving you full control over volume and reputation. Contact support@onesignal.com to learn more about dedicated IPs.

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