Email Auto Warm Up

How to automatically warm your email subdomain within OneSignal.

The ability for an Email to make it into your recipient's inbox is heavily influenced by the reputation of your domain and IP address. Inbox Service Providers (e.g., Gmail, Yahoo, or Outlook) use these reputations to decide whether to deliver your emails or mark them as spam.

Consistent email practices are crucial to establishing yourself as a trustworthy sender. OneSignal aids in this process through a unique technique we created, called Auto Warm Up.


We recommend you send with Auto Warm Up

Anytime you are increasing your sending volume, you should warm up your sending domain to increase your sends incrementally over time and protect your Sender reputation.

Select Auto Warm Up when sending to a Large Audience

When sending an email to a large audience, select "Send with Auto Warm Up" as your Delivery Schedule method.

Send Emails with Auto Warm Up.

Send Emails with Auto Warm Up.

You'll see an explanation of the sending schedule and timing calculated based on the number of recipients in your audience.

What is Email Warm Up?

Email warm up is needed when a new domain is used to either start sending emails or when sending a marketing message to a large audience. It involves gradually establishing a positive reputation for a new or inactive sending domain over time.

During email warm up, you gradually increase the volume of emails sent over a period of time. This builds trust with Inbox Service Providers (ISPs) and ensures that your emails are less likely to be flagged as spam.

The goal of email warm up is to establish a positive reputation for a new or inactive sending domain. Email providers closely monitor the sending behavior of domains to determine whether they are legitimate or potential sources of spam.

When warming up your new domain, we recommend that you:

  • Select an audience of highly engaged recipients with validated email addresses.
  • Make sure your emails have accessible content with more text than images.
  • Focus on engaging and non-promotional content in the initial emails.
  • Monitor metrics such as open rates, click-through rates, and spam complaints.


Warming up your domain is essential for ensuring recipients receive your emails and that they don’t end up in the spam folder.

The benefits are:

  1. reduce the number of emails landing in the spam folder,
  2. enhanced deliverability,
  3. establish trust with ISP, and
  4. reduced inbox bounce rates.

With OneSignal, you don't have to learn the intricacies of how to create a Warm Up campaign.

Sending with Auto Warm Up

How to Select Auto Warm Up

  1. Compose Your Email: Start creating your email message as usual.
  2. Choose Delivery Schedule: In the message page, look for the "Delivery Schedule" section.
  3. Select Auto Warm Up: Within the "Delivery Schedule" section, you'll find the option to "Send with Auto Warm Up".
Send Emails with Auto Warm Up.

Select Auto Warm Up

Viewing the Sending Schedule

Once you've selected 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.

  • Access the Schedule: Click Edit Auto Warm Up to view all Auto Warm Up Schedules and to customize the schedule.
  • Understanding the Schedule: The schedule shows a gradual increase in email volume, tailored to optimize deliverability and sender reputation.

After sending, you can monitor the status of sending by viewing the scheduled and total sent messages on the delivery report and statistics graph.

Auto Population of Recommended Schedule

OneSignal intelligently calculates your initial sending volume based on your email activity in the past 3 weeks. This ensures a smooth and effective warm up process. Our recommended schedule incrementally increases sending to your audience by 20% a day. 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.

Graph displaying send volume over time based on an auto-recommended schedule for a warm up email, based on total audience size.

Graph displaying send volume over time based on an auto-recommended schedule for a warm up email, based on total audience size.

Customize Your Warm Up Schedule


Use the recommended schedule

OneSignal provides a recommended warm up schedule based on best practices. We advise sticking to this schedule for optimal results.

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.
Customize the sending schedule of your Warm Up emails.

Customize the sending schedule of your Warm Up emails.

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.

Auto-complete schedule

Auto-complete schedule

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.


  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.

Recommended Sending Time

Recommended Sending Time


Avoid scheduling messages too far in advance.

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.

Sending Your First Auto Warm Up Email

  1. Prepare Your Email Content: Focus on engaging, non-promotional content with a balance of (mostly) text and images.
  2. Select Your Audience: Choose a segment of your audience that is most likely to engage.
  3. Schedule the Email: Set your email to send using the Auto Warm Up feature.
  4. Monitor Performance: After sending, keep an eye on key metrics such as failures and bounces.


We are always here to help.

If you have any questions about your warm up schedule, please reach out for advice.

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.

πŸ“ˆ This approach ensures a gradual increase in volume, aligning with best practices for Auto Warm Up.

Scheduling Your Second Email

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

  • πŸ•’ 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.


Customize your schedule

You are more than welcome 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 [email protected].

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".


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

Yes, it's advisable to warm up each subdomain separately, even if the main domain has a good reputation. Each subdomain is treated uniquely by ISPs.

Why do I need to send more than one Warm Up Email?

Having multiple warm up email messages is necessary if your first warm up email message only gets you to half the desired volume

Multiple email messages ensure a gradual and steady increase in email volume, which is crucial for building a solid sender reputation without triggering spam filters.

For example, if you're trying to warm your domain to your full audience volume, you might have to send out multiple emails to your full audience because each warm up email is distributed over many days.

What is the difference between Domain & IP Warm Up?

Domain Warm-Up

  • Necessity: Domain warm up is essential whenever you migrate to a new email platform.
  • Purpose: Gradually building a positive reputation for your new or inactive email domain.
  • Process: Involves sending emails in increasing volumes over time to establish trust with Inbox Service Providers (ISPs).
  • Target Audience: Particularly crucial for new customers transitioning to a platform like OneSignal.
  • OneSignal Advantage: 🌟 OneSignal can automate this process, making it seamless and efficient for users to establish a strong domain reputation without the hassle.

IP Warm-Up

  • When It's Needed: This is required if you're using dedicated IP addresses that haven't previously been warmed up.
  • Focus: Centers on establishing a good reputation for your email server's IP address.
  • OneSignal Advantage: 🌟 _OneSignal automates this process in the background. We offer dedicated IPs and can help manage the warming process on your behalf. Reach out to us to inquire about dedicated IPs.

OneSignal vs. Bring Your Own ESP

  • With Third-Party ESPs (e.g., SendGrid, Mailchimp, Mailgun): You're responsible for managing both your IP address and domain reputation.
  • With OneSignal Email Delivery: We manage your IP reputation, but ensuring your domain is 'warm' and has a good reputation remains your responsibility.


Leverage OneSignal's automated features to simplify your entire warm up process.

Considerations for High-Volume Senders

  • Transitioning Platforms: High-volume senders typically start a new subdomain when moving to another platform, as using the same subdomain across different ISPs can lead to DNS issues.
  • Avoiding Complications: Starting fresh with a new subdomain for warming is often simpler than transferring an established domain, especially to avoid disruptions in email sending during the transition.


OneSignal's automation tools are particularly beneficial for high-volume senders, by streamlining the transition and warm up phases.

What's our recommended Warm Up Schedule?

This is the schedule to follow for sending emails that will increase the likelihood of deliverability and reduce ISPs' greylisting, rate-limiting, or blocking your domain or IP address.

StageTotal email sends / day
1620% increase in send volume daily



Follow a 20% daily increase for your send volume from day one. If you have a period of significantly lower sending, and you want to send a message to a larger segment, we recommend lowering your sending amount to align with your recent sending habits, and then increasing it again by 20% a day.

Another example schedule is:
If your highest send volume in the last 21 days is 700, and you want to hit an audience of 2000 recipients, your schedule should look like:


Why are only some of my emails failing?

When emails fail during warmup, this can indicate the volume is too high for the domain's current reputation with the ISPs. Typically, this failure reason surrounding this issue would be indicated by a 602(too old) error.

You may notice that the failure only occurs with certain ISPs such as Google or Outlook. This would also be a characteristic of throttling from the ISP due to reputation.

If you are seeing the 602 error within the Audience Activity failures, the recommendation would be to pull back your volume to the amount that was previously successfully delivered and slowly increase daily.

How can I edit an email that is being warmed up?

Editing is not yet available if your email warm up send has already started. To cancel a sending or scheduled email message, navigate to the email index, click on the three dots on the right side of the row to open up options, and select "Cancel". The status of the email will update to Canceled.

How do I A/B Test my Auto Warm Up email?

You will need to create two separate email messages with different segments and customize the schedule of both. If you're looking to send a 50/50 split AB test, Create two segments of the appropriate size, then for each email, select auto warm up and customize the start date and sending volume for each stage so that they sum to recommended values.

Dedicated IP vs Shared IP

By default, your messages are sent from a shared IP address that helps maintain, which helps up manage your IP warm up on your behalf.

You can also set up a dedicated IPβ€”an IP address for sending email employed by a single user or account. Since only your email will be sent from that IP, only you will determine the volume of mail and the reputation of the IP. To get a dedicated IP, reach out to [email protected] to learn more.