Is it okay to send emails from two IP addresses with the same subdomain concurrently?
Matthew Whittaker
Co-founder & CTO, Suped
Published 7 May 2025
Updated 21 Aug 2025
8 min read
Migrating email sending from one platform to another is a common scenario, but it often brings up complex questions about maintaining deliverability and sender reputation. One such question, and a very pertinent one, is whether it's acceptable to send emails to the same users from two different IP addresses concurrently while using the same subdomain.
We often hear from implementation managers on the vendor side who advise against this practice. Their concern typically stems from potential deliverability issues and the risk of confusing mailbox providers. While their caution is understandable, the reality is more nuanced than a simple 'yes' or 'no' answer.
Understanding the technical underpinnings and strategic implications is crucial. Successfully navigating a migration while maintaining sender reputation (and avoiding getting your domain or IP blocklisted) involves careful planning and precise execution of your DNS records, sending practices, and audience management. It's a delicate balance between technical configuration and strategic operational changes.
Understanding the risks of concurrent sending
The primary concern with sending from multiple IP addresses using the same subdomain concurrently revolves around sender reputation. Mailbox providers, such as Google, track sending behavior associated with both your IP address and your domain (or subdomain). When the same emails or similar content originate from multiple, previously unrelated IP addresses that share a single subdomain, it can raise red flags. This practice is sometimes referred to as 'snowshoe spamming,' where spammers attempt to evade detection by distributing their volume across many IPs.
Mailbox providers maintain separate reputations for IP addresses and domains. While a subdomain helps to isolate the reputation of your marketing or transactional emails from your main domain's corporate communications, simultaneously using two different IPs for that same subdomain can dilute or confuse this reputation. It makes it harder for ISPs to accurately assess your sending patterns and build trust. This is especially true if the content or recipient lists overlap significantly without proper synchronization, potentially flagging your mail as suspicious.
Beyond reputation, there are significant technical hurdles. DNS records like SPF (Sender Policy Framework) specify which IP addresses are authorized to send email on behalf of your domain. You can only have one SPF record per domain. If you attempt to define multiple SPF records for the same sending hostname, your emails will likely fail SPF authentication, resulting in a 'PermError.' This can lead to emails landing in spam folders or being rejected outright.
Similarly, DKIM (DomainKeys Identified Mail) uses a unique selector in its DNS record to verify email authenticity. When migrating to a new email service provider (ESP), you'll typically need to configure new DKIM records with different selectors. While you can have multiple DKIM records for a subdomain, ensuring proper alignment and avoiding conflicts between the old and new ESPs during a transition period is critical to prevent authentication failures and protect your subdomain reputation.
Strategic migration approach for deliverability
Despite the risks, sending from both IPs during a migration is often necessary and can be done safely with a well-planned ramp-up and ramp-down strategy. Instead of a sudden cut-over, a phased approach allows mailbox providers to gradually recognize the new IP addresses associated with your established subdomain reputation. This involves slowly increasing email volume on the new IP(s) while simultaneously decreasing volume on the old ones, ensuring a smooth transition.
A crucial part of this strategy is avoiding duplicate sends to the same recipient from both platforms. Implementing robust suppression lists and managing user segments carefully prevents overloading recipients, which can trigger spam complaints and negatively impact your sender reputation. For instance, if a user is migrated to the new system, they should immediately be suppressed from receiving emails from the old one, even if the campaign content differs. This helps you avoid emails going to spam.
Prioritize migrating high-engagement transactional emails first, such as password resets, welcome emails, or order confirmations. These emails typically have high open and click rates because recipients expect them. Sending these from the new IP addresses helps to quickly build a positive sending history with mailbox providers. As the new IP gains trust, you can then begin migrating your promotional or marketing campaigns.
Consider the cold cut-off date with your old vendor. If you have a firm deadline, it's wise to start your IP warming process well in advance. This allows for a more relaxed transition period and reduces the pressure to send high volumes from an unwarmed IP. It also mitigates the risk of losing CRM performance by prematurely suppressing a large segment of your engaged users before the new platform is fully operational. We've seen businesses allow for 6 to 18 months for large migrations, though it can often be done in a shorter timeframe with proper planning.
Technical considerations for concurrent sending
DNS configuration
The key to concurrent sending lies in meticulous DNS configuration. For SPF, you must include all authorized sending IP addresses or mechanisms (e.g., include:oldesp.com, include:newesp.com) within a single SPF record for your subdomain. If the SPF record exceeds the 10 DNS lookup limit, this can lead to authentication failures.
Return-Path and DKIM
Each ESP will likely use its own Return-Path (bounce) domain. This domain must be unique per ESP for the same subdomain to ensure bounces are handled correctly. For DKIM, ensure each ESP uses a distinct DKIM selector. This is how mail servers differentiate between multiple valid DKIM signatures for the same sending domain.
One of the reasons subdomains are so powerful in email sending is their ability to point to different IP addresses while maintaining the same root domain. This allows for segmentation of your email traffic. For example, you can have different subdomains for different email types (e.g., marketing.yourdomain.com and transactional.yourdomain.com), each with its own IP or set of IPs and thus its own sender reputation. In a migration scenario, however, you're looking to keep the same subdomain but shift the underlying IP addresses.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) plays a critical role in managing this transition. A DMARC record, when configured correctly, instructs receiving mail servers on how to handle emails that fail SPF or DKIM authentication for your domain. More importantly, it provides valuable aggregate and forensic reports that show you which IPs are sending mail on behalf of your domain and how they are performing. This visibility is indispensable during a migration, allowing you to monitor the success of your IP warming and ensure that the old IP is no longer sending mail once the transition is complete. Understanding Google's guidelines for DMARC is key.
Lastly, consider the impact on link tracking and unsubscribe management. Each ESP typically re-writes links to track clicks, and each handles unsubscribe requests internally. If you are sending concurrently, you must ensure that your unsubscribe lists are synchronized daily between both platforms. Otherwise, a user could unsubscribe from one platform and still receive emails from the other, leading to frustration and potential spam complaints. Similarly, confirm with your old vendor how they handle click tracking and redirects after your contract ends to avoid broken links in older emails.
Comparing vendor advice with practical experience
The vendor's perspective
Some vendors may advise against concurrent sending due to the complexities involved in managing DNS, return paths, and deliverability during a transition. They might prefer a clean cut-over to simplify their own setup and prevent potential issues that arise from shared responsibility or misconfigurations. Their advice may be influenced by their platform's capabilities or a desire to prevent issues stemming from technical misconfigurations, such as SPF record conflicts or bounce handling errors.
Potential deliverability pitfalls
Snowshoe spamming: Sending the same or similar content from multiple IPs with a single subdomain can trigger spam filters.
Reputation dilution: Mailbox providers may struggle to assign a consistent reputation to your subdomain if traffic is split between unknown IPs.
Broken authentication: Incorrect SPF or DKIM setup can lead to emails failing authentication checks, hurting deliverability.
Our perspective
While challenging, concurrent sending with the same subdomain and different IPs during a migration is feasible and often the most practical approach. A gradual ramp-up and ramp-down strategy is often the safest path. It minimizes disruption to your email program and allows time for the new IPs to build a solid reputation before the old ones are decommissioned. This transition period helps maintain deliverability throughout the process.
Best practices for success
Gradual ramp-up/down: Slowly shift traffic to the new IP while reducing volume on the old, rather than an abrupt switch.
Unique DNS entries: Ensure distinct bounce domains and DKIM selectors for each ESP.
Synchronized suppression lists: Maintain a unified unsubscribe list across both platforms to prevent duplicate sends.
Final thoughts on managing your email migration
In conclusion, sending emails from two IP addresses with the same subdomain concurrently during an ESP migration is technically possible and often advisable. The vendor's recommendation to avoid it might be a simplification to prevent misconfigurations, or it could be specific to their platform's limitations. However, with careful planning and execution, you can manage the transition effectively.
Focus on a gradual ramp-up, precise DNS configuration, vigilant DMARC monitoring, and robust unsubscribe management. This approach allows you to leverage your established subdomain reputation while warming up new IPs, ensuring minimal disruption to your email program and maintaining strong deliverability throughout the migration.
Views from the trenches
Best practices
Implement a gradual ramp-up and ramp-down strategy for email volume on new and old IPs.
Ensure unique return-path (bounce) domains and DKIM selectors are configured for each ESP.
Actively manage and synchronize suppression lists between both platforms to prevent duplicate sends.
Common pitfalls
Attempting a hard cut-over, which can severely impact deliverability on the new IP due to lack of reputation.
Failing to synchronize unsubscribe lists, leading to spam complaints and recipient frustration.
Creating multiple SPF records for the same hostname, causing authentication failures and rejections.
Expert tips
Consider engaging a deliverability consultant for complex migrations to ensure all technical and strategic nuances are covered.
Thoroughly test all email streams and DNS configurations before ramping up full volume.
Communicate proactively with your new ESP's deliverability team to align on the migration strategy and best practices.
Expert view
Expert from Email Geeks says sending the same content across two different platforms concurrently could raise concerns about snowshoe spamming, but the duration of a typical warm-up might not be long enough for it to become a significant problem.
August 10, 2024 - Email Geeks
Expert view
Expert from Email Geeks says the vendor's advice against using the same subdomain on two platforms concurrently might stem more from technical conflicts, like DNS setup or inbound reply handling, rather than purely strategic deliverability concerns.
Google, track sending behavior associated with both your IP address and your domain (or subdomain). When the same emails or similar content originate from multiple, previously unrelated IP addresses that share a single subdomain, it can raise red flags. This practice is sometimes referred to as 'snowshoe spamming,' where spammers attempt to evade detection by distributing their volume across many IPs.
