How much does subdomain reputation affect core domain reputation?

Key findings

• Varied MBP handling: Mailbox Providers employ diverse methods for assessing domain and subdomain reputation. Some aggregate all reputation data to the root domain, others maintain strict separation, and some use a weighted approach where subdomain performance influences the core domain.
• Inheritance of reputation: A subdomain can inherit age and some initial reputation from its main domain, which can be beneficial, particularly since newer domains are often viewed with suspicion. However, this also means negative aspects can flow both ways.
• Isolation benefits: Using subdomains allows for the isolation of sender reputation, meaning issues like spam complaints or blocklistings on one subdomain are less likely to severely impact the root domain or other subdomains dedicated to different email types, such as transactional vs. marketing emails.
• Cyclical relationship: There's a dynamic relationship where subdomains build their own primary reputation, which then contributes to the parent domain's overall standing. Conversely, the overall parent domain reputation can also influence the reputation of its individual subdomains.
• Blocklist behavior: While MBPs might differentiate, many blocklists (or blacklists) operate at the parent domain level. This means a subdomain issue could still lead to the entire root domain being listed, regardless of the individual subdomain's specific problem.

Key considerations

• Strategic use of subdomains: Employing distinct subdomains for different email categories (e.g., transactional, marketing, alerts) is a widely adopted best practice. This helps silo off reputation and mitigate risks.
• Focus on content and engagement: Rather than attempting to manipulate filtering systems, prioritize sending relevant and engaging content to recipients. This is the most sustainable way to build a positive domain reputation.
• Understanding MBP specifics: While exact algorithms are proprietary, tools like Google Postmaster Tools can offer insights into how specific providers view your domain and subdomain reputation.
• Warming considerations: If using numerous subdomains, especially for a platform sending on behalf of multiple clients, the process of warming each subdomain can be a significant logistical challenge. The degree of isolation provided might not always justify the warming effort if the core domain truly holds most of the weight.

Key opinions

• Isolation benefits: Marketers frequently aim to understand the degree of isolation offered by subdomains, seeking to prevent a poor-performing email stream from damaging the reputation of other critical communications or the main brand.
• Brand protection: The consensus is that using at least one email subdomain is a good practice to protect the root domain's reputation from potential deliverability issues associated with specific sending activities.
• Complexity of MBP rules: There is a recognized challenge in knowing how different Mailbox Providers specifically weigh subdomain versus core domain reputation, as this information is typically proprietary.
• Impact of negative engagement: Concerns are raised about whether negative email engagement on a subdomain will inevitably affect the primary domain's standing.

Key considerations

• Scalability for platforms: Platforms sending on behalf of many clients face significant hurdles in requiring each client to warm their own subdomain, prompting a need to understand the true isolation benefits versus the effort involved.
• Subdomain purpose: Marketers recognize that using a subdomain allows them to build a distinct reputation for specific campaign types, which can ultimately improve deliverability for those campaigns.
• Risk assessment: While subdomains offer a buffer, marketers consider the risk that a compromised subdomain could still negatively affect the broader brand reputation, despite the isolation attempt.
• Avoiding suspicious patterns: Marketers should avoid creating too many similar, unrelated domains in an attempt to spread volume, as this can appear suspicious to filtering systems and human reviewers, signaling a desire to hide problematic sending practices.

Key opinions

• MBP discretion: Experts agree that how much weight is placed on subdomain versus core domain reputation is entirely up to the individual Mailbox Provider, and these configurations are not publicly disclosed.
• Focus on good sending: The primary recommendation is to concentrate on sending desired and engaging content to recipients, rather than attempting to manipulate data streams or systems.
• Multifaceted reputation: Reputation is not a single metric but a complex interplay of various factors, including individual subdomain performance and the overall health of the parent domain.
• Isolation through authentication: Using separate DKIM domains or selectors for different publishers or email types is advised to provide a degree of isolation, even if complete separation is elusive.
• Parent domain influence: While subdomains develop their own reputation, the overall parent domain reputation can still drip down and drive the reputation of each subdomain.

Key considerations

• Near impossible to fully isolate: Achieving complete isolation between customers or email streams is extremely challenging without dedicated IPs and custom reverse DNS.
• Blocklist nuances: Blocklists often target the parent domain, irrespective of individual subdomain performance. To avoid blocklisting, using entirely different, unrelated domains is sometimes suggested, but this can raise suspicion if not managed carefully.
• Proportional reputation model: While not a factual statement, a useful model suggests that if one subdomain sends significantly more mail, its reputation will be proportionally weighted more heavily when calculating the parent domain's overall standing.
• Importance of unique authentication: For individual clients or distinct sending activities, using their own domains with unique email authentication and list management is generally the best strategy for isolation, even if minor issues may still occur.

Key findings

• Reputation siloing: Documentation confirms that a primary benefit of using subdomains is the ability to silo off reputation away from the core branded root domain, minimizing impact from specific sending streams.
• Inheritance of age: Subdomains inherit the age of the main domain, a crucial factor since many email providers view newer domains with suspicion.
• Isolation from issues: If a subdomain experiences issues like spam complaints or blocklisting, it is generally less likely to impact the main domain than if a single domain were used for all email activities.
• Domain reputation centrality: Domain reputation is a primary factor affecting email deliverability, with the branded website taking precedence in ISP filtering decisions.

Key considerations

• Maintaining a distinct reputation: Using subdomains helps in building a distinct reputation for different email types, which can positively impact the deliverability of various marketing campaigns.
• Risk of damage: Subdomains can benefit from the brand's reputation but also risk damaging it if the communication streams associated with them are compromised.
• Improving overall deliverability: Subdomains are cited as a way to significantly improve email deliverability by isolating different types of email traffic, which helps maintain a good sender reputation across the board.
• Protecting the root domain: It is recommended to have at least one email subdomain to protect the root domain's reputation from being adversely affected by potential deliverability issues.