What are the best practices for warming up subdomains after an ESP migration for multi-brand operations?
Michael Ko
Co-founder & CEO, Suped
Published 19 Jun 2025
Updated 26 Aug 2025
9 min read
Migrating to a new email service provider (ESP) is a significant undertaking, especially for multi-brand or multi-location businesses that rely on subdomains for their email sending. We know the challenges this presents, as establishing a new sending infrastructure while maintaining email deliverability is critical. The use of subdomains can actually be a benefit here, allowing for a phased transition rather than an abrupt switch, as both your old and new ESPs can operate concurrently. However, this advantage hinges on a well-executed warm-up strategy for each subdomain.
Properly warming up your subdomains ensures that mailbox providers recognize your sending patterns as legitimate, helping you avoid the spam folder and maintain a strong sender reputation. Without a systematic warm-up, even highly engaged recipients might not receive your crucial communications. This guide aims to outline the best practices for navigating this complex process, ensuring a smooth transition and consistent inbox placement across all your brands.
It's not merely about sending emails, it is about building trust with internet service providers (ISPs) under your new ESP's infrastructure. Every subdomain acts as a distinct sending entity in the eyes of these providers, so each requires its own careful cultivation of sender reputation.
Failing to plan adequately can lead to severe deliverability issues, including emails being sent to spam or even outright blocked. By following a structured warm-up, you can ensure your multi-brand operations continue to reach their intended audience effectively.
Understanding subdomain warming
Subdomain warming is the controlled process of gradually increasing email sending volume from a new or recently migrated subdomain to establish a positive sending reputation with ISPs. Unlike the main domain, a subdomain acts as a separate entity for email sending, meaning its reputation must be built independently, even if the parent domain has a solid standing.
This process is crucial because ISPs monitor various factors to determine if your emails are legitimate or spam, including sending volume, consistency, engagement rates, and complaints. A sudden surge in email volume from a new or unfamiliar subdomain can trigger spam filters, leading to poor inbox placement or even blacklisting (blocklisting). You can learn more about how email blacklists actually work in our guide.
When migrating ESPs, even if you keep the same domain, a new ESP often means new sending IPs. While IP warming is necessary for dedicated IPs, domain warm-up is always critical, especially for subdomains. ISPs need to see consistent, positive sending behavior associated with that specific subdomain. This is particularly relevant for multi-brand setups where each brand might use its own subdomain, such as marketing.brandA.com or transactional.brandB.com.
The goal is to gradually introduce your new sending infrastructure to ISPs, allowing them to build a positive reputation for your subdomains. This foundation of trust is essential for long-term deliverability success, preventing your emails from being flagged as suspicious due to sudden changes in sending patterns or volume.
Planning your multi-brand warm-up strategy
A successful warm-up for multi-brand operations begins with a meticulous plan. Each subdomain needs a tailored strategy based on its sending volume, audience engagement, and message type (transactional versus marketing). The core principle is to start with low volumes of highly engaged users and slowly increase volume over several weeks, carefully monitoring performance.
Critical to this strategy is ensuring your DNS records (SPF, DKIM, and DMARC) are correctly configured for each subdomain on your new ESP. These authentication protocols are fundamental for verifying your identity as a sender and are heavily weighed by ISPs. Without proper authentication, even a perfectly executed warm-up schedule will fall short.
Key warm-up considerations
Audience segmentation: Prioritize your most engaged subscribers who are likely to open and click. This sends positive signals to ISPs.
Content consistency: Send engaging, relevant content during the warm-up. Avoid drastic changes in email type or subject lines.
Volume increment: Gradually increase your sending volume, typically doubling every few days, but adjust based on performance.
Monitoring: Closely watch your deliverability metrics, including open rates, click-through rates, bounces, and complaints.
List hygiene: Ensure your lists are clean and free of unengaged or invalid addresses to minimize bounces and spam traps.
Remember that each subdomain (e.g., news.yourbrand.com, alerts.yourbrand.com) might have different sending volumes and engagement patterns. A cookie-cutter approach across all subdomains is rarely effective. Tailor your warm-up schedule to the specific needs and history of each sending entity.
Executing the warm-up process
The execution phase requires consistency and vigilant monitoring. Begin by setting up your new ESP for sending on the first subdomain. Start by sending to a small, highly engaged segment of your audience. These should be subscribers who have recently opened or clicked your emails. Their positive engagement is critical for signaling good sender behavior to ISPs.
Day
Volume
Target Audience
Monitoring Focus
1-3
1,000-5,000
Most engaged (past 30 days)
Bounces, spam complaints
4-7
5,000-10,000
Recently engaged (past 60 days)
Open rates, soft bounces
8-14
10,000-25,000
Engaged (past 90 days)
Deliverability, inbox placement
15-30
25,000+
Broader engaged segments
Overall performance, domain reputation
As you gradually increase volume, closely monitor your email deliverability metrics. Keep an eye on your open rates, click-through rates, bounce rates, and crucially, your spam complaint rates. Any sudden spikes in bounces or complaints are red flags that indicate you need to slow down or reassess your sending strategy. Remember, the goal is to build a steady, positive reputation.
For multi-brand operations, this process applies to each sending subdomain. If Brand A uses emails.brandA.com and Brand B uses comm.brandB.com, each subdomain requires its own warm-up schedule, list segmentation, and monitoring. This ensures that a reputation issue with one brand doesn't necessarily impact the deliverability of another.
Monitoring and optimization for multi-brand success
Continuous monitoring and optimization are non-negotiable for maintaining stellar email deliverability, especially after a migration. You should regularly check your domain reputation through tools like Google Postmaster Tools and your ESP's analytics dashboards. Pay close attention to spam complaint rates, bounce rates, and engagement metrics (opens and clicks) for each subdomain.
Pre-migration monitoring
Historical data: Baseline performance metrics for each subdomain to compare against post-migration.
List health: Identify and clean unengaged subscribers and invalid addresses.
Post-migration optimization
Continuous adjustment: Modify sending volumes or segmentation based on real-time deliverability feedback.
Feedback loops: Integrate ISP feedback loops (FBLs) to quickly identify and remove complainers.
Should you encounter any deliverability issues, like a subdomain appearing on a blocklist or experiencing a sudden drop in inbox placement, investigate immediately. Potential causes include sending to unengaged lists, high spam complaint rates, or incorrect DNS configurations. Our article on common deliverability issues during warmup can provide further insight.
Remember, email deliverability is an ongoing process. Even after a successful warm-up, continuous monitoring and adherence to best practices are vital for long-term success across all your brands and subdomains. Regular list cleaning, consistent sending, and engaging content will always be your best allies.
These protocols tell receiving mail servers that emails from your subdomain are legitimate and have not been tampered with. Without them, even well-intentioned emails can be flagged as spam or rejected outright. Incorrect configurations, such as exceeding the 10-lookup limit for SPF records, can severely impact deliverability.
Example SPF record for a subdomainDNS
sub.yourdomain.com. IN TXT "v=spf1 include:_spf.newesp.com ~all"
Each of your multi-brand subdomains may require a unique set of these records, especially if different ESPs or sending services are used across brands. Ensure that your DNS provider is updated with the correct records provided by your new ESP. Regularly verifying these records is also a good practice, as accidental changes can quickly lead to authentication failures.
Maintaining a clean and engaged email list
Maintaining a clean and engaged email list is one of the most impactful strategies during and after an ESP migration. Sending emails to inactive, invalid, or spam trap addresses can quickly erode your sender reputation, regardless of how well you’ve warmed up your subdomains. For multi-brand operations, this means meticulously cleaning lists for each brand individually.
Before starting any warm-up, identify and remove all unengaged subscribers, hard bounces, and known spam traps from your lists. While it might seem counterintuitive to reduce your audience, sending to a smaller, highly responsive list during the warm-up period is far more beneficial for reputation building. Your ultimate goal is to reach the inbox, not just send a large volume of emails. Our guide on how to improve domain reputation provides additional tactics.
Best practice: Segment and re-engage
For each brand's subdomain, segment your audience based on engagement levels. Start your warm-up with your most active subscribers, then gradually expand to less active segments. Consider a re-engagement campaign for dormant subscribers, using a separate subdomain if possible, before including them in your main sending streams. This helps to gauge their current engagement without risking your primary sending reputation.
Remember, the quality of your list directly impacts the success of your subdomain warm-up and overall email deliverability. Investing time in list hygiene pre-migration will pay dividends in sustained inbox placement and sender reputation for all your multi-brand operations.
Views from the trenches
Best practices
Always segment your audience by engagement and prioritize your most active subscribers during the initial warm-up phase.
Maintain consistent sending frequency and content type to avoid triggering spam filters with unusual patterns.
Rigorously clean email lists before migration, removing inactive users, bounces, and potential spam traps for better deliverability.
Use separate subdomains for different email types, such as transactional and marketing, to isolate their reputations.
Continuously monitor all relevant email deliverability metrics for each subdomain to detect and address issues promptly.
Common pitfalls
Attempting to send full volume too quickly from new subdomains can lead to immediate blocklisting and poor sender reputation.
Neglecting to configure SPF, DKIM, and DMARC properly for each new subdomain will result in authentication failures.
Failing to clean email lists before migration often leads to high bounce rates and spam complaints during warm-up.
Not having a detailed, phased warm-up plan can cause inconsistent sending patterns and unpredictable deliverability.
Ignoring the importance of unique warm-up schedules for each subdomain in a multi-brand setup can jeopardize overall performance.
Expert tips
Consider a dual-sending approach during migration, where both old and new ESPs send simultaneously from different subdomains for a smoother transition.
Leverage engaged subscribers to help build a positive sending reputation quickly during the early stages of warm-up.
Implement a robust feedback loop system with ISPs to get immediate notifications on spam complaints and manage them effectively.
Educate your team on the importance of the warm-up process to ensure everyone adheres to the new sending guidelines.
Regularly review your DMARC reports for each subdomain to catch authentication errors and potential spoofing attempts.
Expert view
Expert from Email Geeks says that using subdomains is beneficial because it allows for a gradual transition between ESPs. This means both old and new ESPs can send simultaneously, preventing a hard handoff that could disrupt email deliverability.
2024-08-16 - Email Geeks
Marketer view
Marketer from Email Geeks emphasizes that the warm-up process for subdomains is a critical area where expert guidance is highly valuable for a smooth ESP migration.
2024-08-16 - Email Geeks
Achieving deliverability success after ESP migration
Warming up subdomains after an ESP migration for multi-brand operations is a nuanced but essential process. It demands careful planning, disciplined execution, and continuous monitoring. By treating each subdomain as a unique sending identity and systematically building its reputation, you can ensure your email programs maintain high deliverability and strong sender trust.
Embrace the phased approach, prioritize engaged segments, and never underestimate the power of robust email authentication and list hygiene. This strategic investment in your email infrastructure will safeguard your multi-brand communications and preserve your invaluable sender reputation.
like Google Postmaster Tools and your ESP's analytics dashboards. Pay close attention to spam complaint rates, bounce rates, and engagement metrics (opens and clicks) for each subdomain.
