How much does subdomain reputation affect core domain reputation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 12 Jun 2025
Updated 16 Aug 2025
7 min read
The relationship between subdomain reputation and core domain reputation is a common point of confusion for email senders. Many believe that isolating email streams on subdomains completely insulates the main domain from any negative impact. While there's a degree of truth to this, the reality is more nuanced.
Mailbox providers (MBPs) like Google, Yahoo, and Microsoft employ sophisticated algorithms to assess sender reputation, and their approach to subdomains can vary. Some systems might keep subdomain reputations largely separate, while others might aggregate data, weighting it based on volume or other factors to inform the core domain's standing. This makes the question of isolation rather complex, as there is no single, universal rule.
The primary goal of using subdomains for email sending is often to compartmentalize different types of email traffic, like transactional versus marketing emails. This strategy aims to prevent issues with one stream from contaminating the reputation of another. However, while subdomains offer a level of protection, they don't provide a complete shield. Understanding these dynamics is crucial for maintaining strong email domain reputation and ensuring your messages consistently reach the inbox.
How mailbox providers measure reputation
Mailbox providers maintain their own proprietary systems for calculating and tracking sender reputation. These systems are often considered secret sauce, meaning they don't publicly disclose the exact weighting given to subdomains versus the core domain. Some providers might treat each subdomain as a distinct entity with its own reputation, while others might roll up all subdomain reputation into the parent domain, albeit with varying weights.
Even when subdomains are tracked separately, there can still be an influence from the parent domain. A well-established, trustworthy core domain can lend some initial credibility to new subdomains, while a core domain with a poor standing might drag down the starting reputation of its subdomains. This creates a cyclical relationship where a subdomain's behavior impacts the parent, and the parent's standing influences the subdomain.
Tools like Google Postmaster Tools offer some insight into how Google tracks reputation for both root domains and their subdomains. While it doesn't reveal the full weighting logic, it does provide valuable data points on spam rates, IP reputation, and domain reputation, which can help you infer how your subdomains are performing in relation to your main domain. Mailgun also offers insights into the role of subdomains in email reputation.
Strategic use of subdomains for isolation
Using subdomains is a widely adopted best practice for managing email deliverability, especially for organizations that send various types of email. The primary advantage is the ability to isolate reputation. For instance, if your marketing emails generate higher spam complaints due to their nature (e.g., promotional content), sending them from a subdomain like marketing.yourdomain.com can prevent this negative feedback from directly affecting the deliverability of your critical transactional emails sent from transactional.yourdomain.com.
Best practice: Isolate email streams
By segmenting your email traffic across different subdomains, you create distinct reputation profiles for each sending type. This means that issues on one subdomain, such as a temporary blocklist (or blacklist) listing or a spike in spam complaints, are less likely to severely impact the deliverability of emails sent from your other subdomains, or your core domain. This approach is highly recommended for protecting your overall brand communication.
Transactional emails: Use a dedicated subdomain like mail.yourdomain.com or t.yourdomain.com for critical messages like password resets, order confirmations, and security alerts.
Marketing emails: Use another subdomain, such as news.yourdomain.com or promo.yourdomain.com, for newsletters and promotional content.
Dedicated subdomains for clients: If you operate a platform sending on behalf of multiple clients, consider assigning each client their own subdomain to isolate their sending reputation.
This segregation prevents issues from one subdomain from negatively impacting the core domain, which is vital for your brand's integrity. For instance, if a marketing campaign unintentionally triggers a high spam complaint rate on promo.yourdomain.com, your password reset emails from mail.yourdomain.com are more likely to bypass spam filters and reach recipients. This is a core benefit of adopting a subdomain strategy.
Understanding reputation leakage and blocklists
While subdomains offer isolation, it's incorrect to assume a complete separation. Reputation can bleed through from a subdomain to its core domain, especially if the issues are severe or persistent. Mailbox providers often employ algorithms that factor in the collective reputation of all subdomains when assessing the overall health of the parent domain. If one subdomain consistently exhibits poor sending behavior, it will inevitably impact the perception of the core domain.
Blocklists (also known as blacklists) are another critical factor where the isolation effect can be limited. Many blocklists operate at the root domain level. This means if a subdomain is listed for spamming or malicious activity, the entire parent domain could be flagged, regardless of the individual subdomain's reputation. This is why blocklist monitoring is essential, as a listing can quickly deteriorate your overall sender reputation and lead to widespread delivery issues.
The purpose of subdomains
Traffic segmentation: Allows for different email types (e.g., transactional, marketing, support) to have their own distinct sending streams and dedicated metrics.
Reputation management: Mitigates the risk of a single problematic email stream negatively impacting your core domain's overall standing.
Algorithmic aggregation: Mailbox providers may still factor subdomain performance into the core domain's reputation, especially for significant deviations.
Shared infrastructure: If subdomains share the same underlying IP addresses, a poor reputation on one can still affect the deliverability of others due to shared IP reputation.
It's also worth noting that using many related but different root domains (e.g., domain1.com, domain2.com) instead of subdomains can raise suspicion among mail administrators and machine learning filters, as it might appear as an attempt to hide questionable sending practices. This could lead to closer scrutiny and potentially worse deliverability outcomes than a well-managed subdomain strategy.
Strategic management and authentication
To protect your core domain's reputation, even when using subdomains, proactive management is key. This includes ensuring proper email authentication for all sending domains and subdomains. Implementing DMARC monitoring on both your core domain and subdomains will provide valuable feedback on authentication failures and potential spoofing attempts, helping you identify and mitigate risks quickly.
Regularly monitor your email deliverability metrics for each subdomain, paying close attention to spam complaint rates, bounce rates, and engagement. If a subdomain's metrics start to decline, it's a clear signal to investigate and adjust your sending practices for that specific stream. This targeted approach helps contain issues before they can spread and significantly impact your main domain. For example, Google provides insights for high spam rates impacting deliverability.
Ensuring proper DKIM signing for each subdomain is also crucial. This allows Mailbox Providers to verify the authenticity of your emails at a granular level. When you're a platform sending on behalf of many, you can sign messages with a different d= tag or selector for each publisher, providing better isolation. For instance, you could use:
DKIM example for Publisher 1DNS
s=123 d=publisher1.yourdomain.com
DKIM example for Publisher 2DNS
s=abc d=publisher2.yourdomain.com
Views from the trenches
Best practices
Actively monitor specific subdomain metrics to quickly identify and address emerging deliverability issues.
Use dedicated subdomains for different email types (e.g., transactional, marketing) to segment reputation.
Implement DMARC with a monitoring policy for both your main domain and all subdomains for visibility.
Maintain strong sender authentication (SPF, DKIM, DMARC) for every subdomain you use for sending.
Ensure content quality and audience engagement are high across all email streams, regardless of the subdomain.
Common pitfalls
Assuming complete reputation isolation between subdomains and the core domain.
Neglecting to monitor subdomain performance, leading to unchecked reputation damage.
Using multiple, unrelated root domains instead of subdomains, which can appear suspicious to MBPs.
Failing to implement proper authentication for all subdomains, making them vulnerable to spoofing.
Ignoring blocklist (or blacklist) listings, as they often apply to the root domain regardless of subdomain.
Expert tips
Mailbox providers' reputation systems are complex and proprietary, so observed behavior is often the best indicator.
Even with subdomains, poor performance can 'bleed' up to the root domain, particularly with high volume or severe issues.
Blocklists frequently list the parent domain, highlighting the importance of overall domain health.
Dedicated domains for each client (if you're a sending platform) offer the best isolation, but subdomains are a strong alternative.
Focus on sending content recipients want, as this is the most powerful factor for long-term deliverability success.
Expert view
Expert from Email Geeks says that how much weight is put on the reputation of subdomains versus the core domain depends entirely on the receiver's configuration, as some roll it all up, some keep them separate, and others use weighted data.
2021-08-09 - Email Geeks
Marketer view
Marketer from Email Geeks says that for platforms sending on behalf of many, using subdomains offers isolation but requires each sender to warm their domain with platform IPs, which is not feasible at scale.
2021-08-09 - Email Geeks
Navigating subdomain and core domain reputation
Subdomain reputation has a significant, albeit complex, impact on core domain reputation. While subdomains offer valuable isolation for different email streams, they are not a foolproof shield against negative reputation spillover, especially concerning how Mailbox Providers (MBPs) aggregate data and how blocklists (or blacklists) operate. By carefully segmenting your email traffic, implementing robust authentication, and diligently monitoring performance across all your sending domains, you can optimize your email deliverability and safeguard your core brand reputation.
Google, 
Yahoo, and 
Microsoft employ sophisticated algorithms to assess sender reputation, and their approach to subdomains can vary. Some systems might keep subdomain reputations largely separate, while others might aggregate data, weighting it based on volume or other factors to inform the core domain's standing. This makes the question of isolation rather complex, as there is no single, universal rule.
