Do you need to warm up an existing email domain on a new ESP and can you use the same DKIM for multiple ESPs?
Matthew Whittaker
Co-founder & CTO, Suped
Published 18 May 2025
Updated 18 Aug 2025
8 min read
Migrating to a new email service provider (ESP) is a significant undertaking for any business. While the promise of enhanced features, better deliverability, or cost savings is appealing, concerns about maintaining sender reputation and ensuring uninterrupted email flow often arise. A common question I hear is whether an existing, well-established email domain needs to undergo a warm up period when moving to a new ESP, especially if the sending volume remains consistent.
The short answer is yes, a warm up or ramp-up is almost always necessary, even for an existing domain. This is because email deliverability is a complex dance between your domain's historical reputation, the IP addresses used for sending, and the specific sending patterns observed by mailbox providers. When you introduce a new ESP, you are introducing a new set of IP addresses and potentially new sending behaviors, which triggers a fresh assessment by recipient mail servers.
Another crucial aspect of ESP migration involves DomainKeys Identified Mail (DKIM). Many wonder if they can use the same DKIM signature across multiple ESPs or if each provider requires a distinct setup. Understanding how DKIM selectors work is key to managing your email authentication effectively and maintaining a strong sender reputation across all your sending platforms.
While your domain already has an established reputation, the specific IP addresses from which your emails originate are new to mailbox providers in combination with your domain. Think of it like moving to a new house. Your personal reputation precedes you, but the neighbors still need to get used to the new activity at your address. Mailbox providers like Google and Outlook meticulously track the reputation of IP addresses and how they behave in conjunction with specific sending domains. A sudden surge in volume from a previously unseen IP-domain pair can trigger spam filters and result in emails being sent to the junk folder or even being blocklisted (blacklisted).
This warm-up process for an existing domain is often referred to as a ramp-up, as it's typically less intense and faster than warming up a completely new domain and IP from scratch. The existing domain reputation acts as a significant advantage. However, ignoring the ramp-up can still lead to deliverability issues. The process involves gradually increasing your sending volume from the new ESP, starting with your most engaged subscribers, to build a positive sending history on the new infrastructure. This helps mailbox providers trust the new combination of your domain and the ESP's sending IPs.
The exact duration and intensity of the ramp-up depend on several factors, including your sending volume, list engagement, and the specific ESP you are using. Some ESPs provide dedicated IPs, while others use shared IP pools. While shared IPs might already have some reputation, your specific domain's sending behavior through that pool is still new. It is always wise to consult your new ESP for their recommended warm-up guidelines.
What changes
Sending IPs: Your emails will now originate from a new set of IP addresses owned by the new ESP. Even on shared IPs, your domain's specific sending history with those IPs is new.
Sending patterns: The new ESP may have different infrastructure, sending speeds, and retry mechanisms. These can influence how mailbox providers perceive your sending behavior.
Technical setup: While you use your existing domain, you'll update DNS records like SPF and DKIM to authorize the new ESP. This is a technical change that recipient servers note.
What remains
Domain reputation: Your existing domain has a history of positive or negative engagement, which provides a foundation. This is why it's a ramp-up, not a cold warm-up.
Subscriber list: The quality and engagement of your email list remain constant, influencing how well your emails are received.
Content: The content of your emails generally stays the same, which is a consistent factor in reputation building.
Using the same DKIM for multiple ESPs
Yes, you absolutely can use the same DKIM d= tag (which represents your domain) across multiple ESPs. DKIM allows for this flexibility through the use of selectors. A DKIM selector is a unique name associated with a specific public key that your ESP uses to sign your emails. When an email is received, the recipient server looks up the DKIM record using both your domain and the selector specified in the email header.
Each ESP you use to send emails from your domain will provide you with a unique DKIM record, which includes a specific selector. For example, ESP A might provide a record with selector s1, and ESP B might provide one with selector espb2024. You simply add both of these DKIM records to your domain's DNS settings. This allows each ESP to sign emails on your behalf while still being authenticated by your domain. This approach is not only permissible but often recommended for maintaining DMARC alignment and a consistent brand identity across all your email streams.
The key is that each DKIM key should have a unique selector within your DNS for that domain. This ensures that when a recipient mail server receives an email from your domain, it knows exactly which public key to retrieve from your DNS to verify the signature. You can have multiple DKIM records for a single domain, each with a different selector, authorizing different senders or ESPs. This setup is common for businesses that use separate ESPs for transactional, marketing, or customer service emails.
By using unique selectors for each ESP, you maintain clear separation and control over your email streams. This also helps in troubleshooting deliverability issues, as you can pinpoint which ESP's DKIM signature might be failing if problems arise. Regularly reviewing your DMARC reports will help you ensure that all your DKIM signatures are authenticating correctly.
Example DKIM records with different selectorsDNS
s1._domainkey.yourdomain.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDqXnFw..."
espb2024._domainkey.yourdomain.com. IN TXT "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA..."
Scenario
DKIM setup
Impact on deliverability
One domain, one ESP
Single DKIM record with one selector provided by the ESP.
Optimal. Clear authentication path, easy to manage.
One domain, multiple ESPs
Each ESP provides a unique DKIM record with its own selector. All records added to domain's DNS.
Optimal. Each ESP is properly authenticated, maintaining distinct reputation and DMARC alignment for each email stream.
One domain, multiple ESPs (shared selector)
Rare and not recommended. If ESPs somehow use the same selector for different keys, it creates conflicts.
Poor. Leads to DKIM failures, DMARC non-compliance, and negatively impacts deliverability. Your emails could be blocklisted (blacklisted).
Best practices for a smooth transition
A smooth transition to a new ESP, especially with an existing domain, hinges on careful planning and execution. Start by segmenting your audience and gradually shifting your email volume to the new platform. Begin with highly engaged subscribers who are likely to open and interact with your emails. This positive engagement helps build trust with mailbox providers and reinforces your sender reputation on the new infrastructure, lessening the chance of your emails hitting a spam trap or being placed on a blocklist.
During the migration, continuous monitoring of your email deliverability is non-negotiable. Pay close attention to your open rates, click-through rates, bounce rates, and spam complaint rates. Tools that offer blocklist monitoring and DMARC reporting can provide invaluable insights into how your emails are performing. Addressing any issues promptly can prevent long-term damage to your sender reputation.
Key migration considerations
Authenticate your domain: Ensure SPF, DKIM, and DMARC records are correctly configured for your new ESP. Incorrect setup is a common cause of deliverability issues.
Gradual ramp-up: Even with an existing domain, increase sending volume slowly to allow mailbox providers to assess your new sending patterns and IP reputation. Find more details on building sender reputation.
Monitor performance metrics: Keep a close eye on delivery rates, bounces, and complaints. Adjust your sending strategy if you notice any dips.
Maintain list hygiene: Clean your list before migrating to avoid sending to inactive or invalid addresses, which can negatively impact your new sender reputation.
Consider setting up a dedicated subdomain for your marketing or transactional emails with the new ESP. This isolates the sending reputation of your main domain from specific email streams. If issues arise with your marketing emails, for example, your corporate emails sent from your primary domain remain unaffected, ensuring business continuity. This strategic separation can be a lifesaver for deliverability.
Moving forward with confidence
Migrating to a new ESP doesn't have to be a daunting task, even with an established email domain. While a form of warm-up (or ramp-up) is almost always necessary, your existing domain reputation provides a strong foundation. The key is to acknowledge the change in underlying sending infrastructure and introduce your new ESP's IPs gradually to mailbox providers.
Furthermore, managing your DKIM records across multiple ESPs is straightforward thanks to the use of unique selectors. This allows you to maintain consistent email authentication and DMARC alignment, crucial for ensuring your emails reach the inbox and are not flagged as spam or blocklisted (blacklisted). Properly configured DKIM helps mailbox providers verify that your emails are legitimate and have not been tampered with in transit.
By following best practices for gradual sending, meticulous authentication setup, and continuous monitoring, you can navigate ESP migration confidently. This proactive approach safeguards your sender reputation and ensures your email programs continue to perform optimally, no matter which ESP you choose to send through.
Views from the trenches
Best practices
Always conduct a ramp-up when switching ESPs, even with an existing domain and shared IPs.
Use dedicated subdomains for different email streams to isolate reputation and mitigate risks.
Ensure SPF, DKIM, and DMARC are correctly configured for each new ESP and monitored closely.
Start sending to your most engaged subscribers first during the warm-up period.
Common pitfalls
Skipping warm-up entirely due to having an existing domain, which can lead to deliverability issues.
Using a personal email address from the parent domain for bulk sending, risking corporate mail.
Not monitoring email performance metrics closely during and after migration, missing critical issues.
Failing to update all necessary DNS records for the new ESP, causing authentication failures.
Expert tips
DKIM selectors are crucial for managing multiple ESPs with the same domain.
Consider the 'mail stream' reputation, not just isolated IP or domain reputation.
Isolating mail streams with subdomains provides a clear advantage for troubleshooting.
A gradual ramp-up allows mailbox providers to adjust to new sending patterns and IP combinations.
Expert view
Expert from Email Geeks says any new combination of domain and IPs should be introduced with at least some ramp-up, even if it is not a full warm-up from scratch.
May 2, 2024 - Email Geeks
Expert view
Expert from Email Geeks says warming up the mail stream is necessary, and while shortcuts can affect how fast you warm up, they do not eliminate the need for warming up.
Google and 
Outlook meticulously track the reputation of IP addresses and how they behave in conjunction with specific sending domains. A sudden surge in volume from a previously unseen IP-domain pair can trigger spam filters and result in emails being sent to the junk folder or even being blocklisted (blacklisted).
