This article explains how to warm up a new sender domain or IP and how to manage the process using segmentation and gradual volume increases in Symplify.
When a new sender domain or sending IP is introduced, mailbox providers need time to evaluate the sender and understand the sending behaviour. This process is known as warm-up.
During the warm-up period, mailbox providers observe sending patterns and how recipients interact with the emails. Gradual increases in sending volume combined with healthy engagement help establish a stable sender reputation.
If sending volume increases too quickly or infrastructure changes suddenly, mailbox providers may apply filtering or reduce inbox placement. A controlled warm-up process helps reduce this risk.
When warm-up is required
Warm-up is typically required whenever a new sending setup is introduced.
This commonly happens when a new sender domain is used for email communication or when a new sending IP is introduced. It may also be necessary when migrating between email platforms or when sending volume increases significantly, for example when expanding into a new market.
Mailbox providers continuously evaluate sending behaviour. They analyse how sending volume develops over time, how recipients interact with emails and whether the domain configuration remains consistent.
Large or sudden changes can appear unusual and may affect deliverability. Gradual growth helps mailbox providers recognise the sender as trustworthy.
Starting with engaged recipients
When beginning a warm-up process it is recommended to first send emails to recipients who are more likely to engage with the messages.
Strong engagement signals that recipients expect and interact with the emails. This helps mailbox providers recognise the sender as legitimate.
A common approach is to begin with recipients who have opened an email within the past three months. This type of engagement-based segmentation ensures that the first sends reach an audience that is more likely to interact with the message.
Example of segment configuration
Below is an example of how this segment can be configured in Symplify.
Example: Segment targeting recipients who have opened any email within the last 12 weeks.
This type of segment ensures that the first warm-up sends are delivered to recipients who recently interacted with previous emails.
Gradually increasing sending volume
After the first sends, the sending volume should increase step by step.
Mailbox providers look for stable growth patterns rather than sudden spikes in activity. Increasing volume gradually allows them to learn that the sending behaviour is consistent and expected.
For many databases the full sending volume can be reached within two to four weeks. The exact pace depends on engagement levels, complaint rates and the overall quality of the recipient data.
If engagement drops or complaint levels increase, it is recommended to slow down the ramp-up until performance stabilises.
Example warm-up configuration in Symplify
In Symplify, a practical way to control warm-up volume is by combining engagement-based segmentation with a shuffle split in a multi-step campaign or journey.
The campaign begins with the engagement segment described earlier. This ensures that only recently active recipients enter the flow.
A shuffle split can then be used to control how much of the audience receives the email.
Example Multi-step configuration
Example: Using a shuffle split to send the warm-up email to 10% of the engaged audience while holding back the remaining recipients.
In the first send, the email may be delivered to around ten percent of the engaged audience. If engagement metrics remain stable, the next send can increase the percentage to twenty or thirty percent.
Over time the share of recipients receiving the email gradually increases until the full segment is included.
This approach allows the sending volume to grow naturally without needing to manually select random sample sizes from the database.
Practical tip
Before flow-based segmentation was commonly used, senders sometimes tried to create warm-up groups by manually selecting random recipients from the database. This could involve filtering on specific values or exporting small recipient lists to control the send volume.
This approach is both time consuming and unreliable.
Using engagement segmentation combined with a shuffle split provides a much more practical way to manage warm-up sends. It ensures that recipients are selected randomly while still allowing the sending volume to increase gradually over time.
Monitoring performance during warm-up
Monitoring performance is an essential part of the warm-up process.
Mailbox providers respond directly to recipient behaviour, so engagement and deliverability metrics must be reviewed carefully during each stage of the ramp-up.
Metrics such as open rate, click rate, bounce rate, spam complaints and unsubscribe rates provide important signals about how mailbox providers are responding to the sends.
If complaint rates increase, bounce rates spike or engagement drops significantly, it is recommended to pause the ramp-up until the issue is resolved.
Because warm-up performance depends on recipient behaviour, customers are responsible for monitoring these metrics and adjusting the ramp-up if performance changes. Note, bounces can be reported many days after sendtime, so the ramp-up should not be rushed.
Migrating sending infrastructure
Warm-up is also relevant when migrating from one email platform to another.
Mailbox providers already recognise the sending behaviour of the existing infrastructure. Switching all sends to a new platform overnight can interrupt the established reputation.
A more stable approach is to run both platforms in parallel while gradually shifting sending volume from the old system to the new one.
During the early stages most emails continue to be sent from the existing system while a smaller share is sent from the new platform. Over time the volume from the new system increases while the volume from the old system decreases.
Maintaining a relatively stable total sending volume during this process helps mailbox providers recognise the infrastructure change without interpreting it as unusual behaviour.
The core principle
Warm-up is ultimately about building trust with mailbox providers.
A gradual increase in sending volume combined with strong recipient engagement helps mailbox providers recognise that the emails are expected and relevant.
In most situations it is safer to scale slightly slower than to increase volume too quickly and risk damaging the sender reputation.