Should I use the same or different subdomains for multiple ESPs?

Key findings

• Reputation isolation: Using separate subdomains for different ESPs or email types allows you to isolate sender reputation. If one email stream encounters deliverability issues or gets placed on a blocklist, it is less likely to negatively impact other streams, protecting your overall domain reputation.
• DMARC alignment: Achieving DMARC alignment for both SPF and DKIM can be simpler with dedicated subdomains for each ESP. While not strictly essential, it can streamline the configuration process for authentication records.
• Complexity vs. simplicity: Sharing a subdomain can simplify DNS management if you're comfortable with the shared reputation risk. However, it can add complexity when troubleshooting authentication issues across different platforms.
• From and reply-to addresses: The subdomain used in your From: header (and often the Reply-To: address) is crucial. Ensure these are consistent with your authentication setup.

Key considerations

• Sender reputation management: Consider the impact of shared reputation. If one ESP sends undesirable mail, it could tarnish the reputation of your shared subdomain, affecting delivery for all mail streams using that subdomain. Using distinct subdomains provides better control over sender reputation.
• Authentication configuration: Ensure that SPF and DKIM records are correctly set up for each ESP using a specific subdomain. This is critical for successful email authentication and deliverability. You can learn more about this in our guide on setting up email authentication for multiple ESPs.
• Email stream segmentation: For optimal deliverability, it's often best practice to segment email types (e.g., marketing, transactional, alerts) to different subdomains. This helps maintain a cleaner sending reputation for critical email flows. This is also covered by industry advice.
• Monitoring and troubleshooting: Managing separate subdomains might require more intricate monitoring, but it can make troubleshooting deliverability issues much more straightforward, as problems can be isolated to a specific stream or ESP.

Key opinions

• Shared reputation risks: Marketers recognize that using the same subdomain across ESPs means that the sending reputation is shared, potentially affecting all email streams if one performs poorly.
• Practical reply handling: For marketers, the ability for customers to easily reply to a consistent email address (using the subdomain) is a key consideration, regardless of the underlying ESP.
• ESP guidance adherence: Many marketers follow their ESP's recommendations for setting up authentication, even if it leads to less conventional configurations like different DMARC reporting addresses.
• Deliverability checks validation: If deliverability checks pass, marketers tend to feel confident in their current setup, even if it might deviate from textbook best practices for specific configurations.

Key considerations

• Simplified management: While using multiple subdomains can be beneficial, marketers often prefer a simpler setup for domain and authentication management if the shared reputation risk is acceptable.
• Consistent branding: Using a single subdomain (or a limited set) can reinforce brand consistency across different types of email communications, which is important for brand recognition.
• Onboarding new ESPs: When onboarding a new ESP, the decision to use an existing or new subdomain impacts the warm-up process and how quickly the new platform can leverage existing domain reputation. For more on this, see building your sending reputation on a new ESP.
• Impact on campaign performance: Marketers must monitor email deliverability rates and engagement metrics to determine if their subdomain strategy is effective, adjusting if necessary to avoid blocklists or spam folders.

Key opinions

• Reputation sharing: Experts acknowledge that using the same subdomain across multiple mail streams means their reputations are more strongly interconnected. Issues with one stream can easily impact the others.
• Authentication importance: It is crucial to have mail authenticated via both SPF and DKIM. Experts stress the importance of DMARC alignment with the From: header for optimal deliverability.
• Simplicity vs. necessity: While different subdomains might simplify achieving DMARC alignment, experts clarify that it's not strictly essential to have separate subdomains for this purpose. It's a 'decent goal' rather than a hard rule.
• DMARC reporting distinctness: Experts advise that DMARC reports (RUA/RUF mailto addresses) should be separate and routed to automation (e.g., an outsourced service) rather than to human-monitored inboxes used for customer replies.
• Recipient's address in To: field: Experts highlight that placing the recipient's email address in the To: field is significantly better for deliverability than not doing so.

Key considerations

• Isolation benefits: For senders with diverse email programs or those concerned about one stream affecting another, using distinct subdomains offers a robust way to isolate reputation and mitigate risks, including those related to blocklists (or blacklists).
• Authentication complexity: While DMARC alignment is a goal, experts advise carefully configuring SPF records and DKIM for each subdomain, as misconfigurations can lead to authentication failures and deliverability issues.
• Strategic subdomain choice: The choice of whether to use the same or different subdomains should align with your overall email sending strategy, considering factors like email volume, content type, and the sensitivity of the mail stream. This is a common topic in deliverability forums.
• Monitoring and reporting: Regardless of the subdomain strategy, robust monitoring of DMARC reports and deliverability metrics is essential. This helps in promptly identifying and addressing any issues related to authentication or reputation across your sending domains and IP addresses.

Key findings

• Multiple subdomain support: Documentation confirms that it is technically possible and often recommended to use multiple subdomains within a single primary domain for various purposes, including email sending.
• Dedicated IP addresses: Subdomains can be used to direct traffic or email streams to different IP addresses, allowing for separation of sending infrastructure and improved traffic management.
• Authentication configuration: Each subdomain requires its own specific DNS records (like CNAMEs for DKIM, or SPF records) to ensure proper email authentication by the respective ESP.
• DMARC and BIMI compatibility: Protocols like DMARC and BIMI are designed to work effectively with subdomains, offering enhanced security and brand display across a range of sending configurations. For example, BIMI Group FAQs clarify logo support across domains and subdomains.

Key considerations

• DNS record management: Proper management of DNS records is paramount. Each subdomain used for sending email needs correctly configured SPF, DKIM, and DMARC records, often provided by your ESP. Refer to best practices for using subdomains.
• Server hosting flexibility: Documentation confirms that subdomains can be hosted on different servers than the main domain. This allows for distributed email infrastructure and specialized hosting for different email types.
• Impact on sender reputation: While subdomains offer reputation isolation, technical documentation advises that improper sending practices on one subdomain can still subtly influence the reputation of the root domain. Consistent good sending practices across all subdomains are vital.
• TXT record for apex domain: Some documentation suggests adding a specific TXT DNS record for the apex domain to ensure multiple projects can use subdomains effectively. This provides a centralized control point for subdomain delegation.