Will sharing a subdomain between two ESPs cause DNS conflicts?

Yes, it is highly likely to cause DNS conflicts. Each ESP requires specific DNS records for authentication (SPF, DKIM), and if one ESP controls the subdomain's NS records, the other ESP may not be able to add or modify its own required entries. This can lead to authentication failures.

How does sharing a subdomain impact sender reputation?

Sharing a subdomain means that the sending reputation of each ESP's email program will affect the other. If one ESP has poor sending practices (e.g., high spam complaints, bounces), it can negatively impact the overall reputation of the shared subdomain, potentially leading to deliverability issues for both.

Is it advisable to use the same subdomain for multiple ESPs?

It is generally not recommended. The technical complexities, potential for DNS conflicts, and risks to sender reputation often outweigh any benefits. It makes managing email authentication and troubleshooting deliverability issues significantly harder.

Can the Return-Path domain be shared between multiple ESPs on the same subdomain?

The Return-Path domain (Mail From) is almost always unique to each ESP for bounce handling and feedback loops. While it might be theoretically possible to share the From (5322.From) domain under very specific conditions, sharing the Return-Path is not feasible.

What is the recommended alternative to sharing a subdomain?

The best practice is to use separate, dedicated subdomains for each ESP. This isolates sender reputation, simplifies DNS management, and makes troubleshooting deliverability issues much clearer and more efficient.

What are the implications of sharing a subdomain between two different ESPs? - Sender reputation - Email deliverability - Knowledge base

The idea of using the same subdomain for sending emails across two different Email Service Providers (ESPs) might seem efficient, especially for organizations managing multiple email programs. At first glance, it appears to simplify branding and domain management. However, this approach introduces a complex web of technical challenges and significant risks to your email deliverability and sender reputation.

From a DNS perspective, sharing a subdomain can lead to direct conflicts, while from a reputation standpoint, it blurs the lines of accountability. It makes troubleshooting deliverability issues incredibly difficult. Let's delve into the specific implications and explore why this setup is generally not recommended, along with safer alternatives.

Navigating DNS and technical configurations

Every ESP requires specific DNS records for proper email authentication, including SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail). When you delegate a subdomain to an ESP, typically by pointing its NS (Name Server) records to the ESP's servers, that ESP gains full control over the subdomain's DNS. This is a common practice to ensure all necessary records are in place for optimal deliverability.

The immediate technical hurdle arises when a second ESP needs to add its own SPF include statements or DKIM CNAME/TXT records to the same subdomain. If the first ESP controls the NS records, you might not have the ability to modify these DNS entries directly. Even if you could, adding multiple SPF include mechanisms can quickly lead to exceeding the 10-lookup limit, which results in SPF authentication failures.

Another critical component is the Return-Path (or 5321.MailFrom) domain. Each ESP typically uses its own unique Return-Path domain to handle bounces and process feedback loop data. It is almost impossible for two distinct ESPs to successfully share the exact same Return-Path setup for a single subdomain, as this is deeply tied to their infrastructure for tracking and reporting. This also means you can't point one subdomain to multiple email service providers for the Return-Path.

Even in scenarios where the primary ESP might allow for manual DNS record additions, coordinating and maintaining SPF and DKIM configurations for multiple platforms on one subdomain is inherently complex. This complexity significantly increases the risk of misconfigurations, which can lead to authentication failures and, consequently, impact your email deliverability. For detailed guidance on this, consider how to configure DNS records to send emails from two different ESPs using the same subdomain.

Impact on sender reputation

While a subdomain does inherit some of its reputation from the root domain, it largely builds its own. Sharing a single subdomain between two ESPs means their sending practices become intertwined, directly affecting each other's sender reputation. If one ESP experiences deliverability issues, such as high spam complaints or bounce rates, the reputation of that shared subdomain will suffer, impacting all emails sent from it, regardless of the ESP origin.

This lack of isolation can lead to one ESP's poor performance dragging down the other's. For example, if a campaign sent via one ESP triggers a high volume of spam complaints, the entire shared subdomain may be flagged by ISPs, potentially resulting in emails from both ESPs landing in spam folders or being blocklisted (or blacklisted). This scenario highlights why it's beneficial to isolate email traffic. You can explore this further by understanding if using a subdomain for different email types affects your primary domain's reputation.

Troubleshooting deliverability problems also becomes considerably more complex. When emails from a shared subdomain face issues, it becomes difficult to pinpoint which sending stream or ESP is responsible for the decline in reputation or a blocklist (blacklist) placement. This requires gathering data from multiple platforms, which often present metrics differently, making a unified analysis challenging. Understanding considerations before sharing sending domains can help clarify the risks.

Strategic choices for domain management

Given the technical complexities and reputation risks, the widely accepted best practice is to use a separate, dedicated subdomain for each ESP. This strategy creates distinct sending environments, allowing each platform to manage its DNS records independently without conflicts and to build its own reputation. For insights into this approach, consider the best practices for using unique or shared email subdomains.

Isolating subdomains offers several benefits:

Reputation Isolation: Poor performance from one ESP will not impact the deliverability of emails sent through another.
Simplified DNS Management: Each ESP can manage its required DNS records without interference.
Easier Troubleshooting: Pinpointing the source of deliverability issues becomes straightforward when each sending stream is distinct.
Clear Reporting: Performance metrics are specific to each ESP, providing clearer insights into campaign effectiveness.

While it's theoretically possible to share a subdomain under extremely specific conditions, such as the primary ESP allowing manual DNS record additions and only sharing the From domain (not the Return-Path), it is rarely advisable. This setup increases the risk of misconfiguration and unexpected changes by either ESP that could disrupt your email program. For robust domain management, delegating a specific subdomain to each ESP via NS records is a sound strategy, giving each provider full control over their sending infrastructure, as detailed in M3AAWG's Best Common Practices for Sending Domains.

Sharing a subdomain

Technical management: Highly complex due to DNS record conflicts (SPF, DKIM) and the 10-lookup limit for SPF. One ESP typically controls the NS records, limiting the other's ability to add necessary entries. This aligns with whether you can use the same sending domain with multiple ESPs.

Reputation impact: High risk of cross-contamination. Poor sending practices from one ESP can negatively affect the entire subdomain's reputation, impacting all email streams.

Troubleshooting: Difficult to isolate the root cause of deliverability issues (e.g., blocklist or blacklist placement) when multiple sources contribute to the same sending identity.

Using separate subdomains

Technical management: Simpler, as each ESP has full control over its designated subdomain's DNS records, eliminating conflicts. This aligns with whether you should use the same or different subdomains.

Reputation impact: Isolated reputation for each sending stream. Issues with one ESP do not affect the others, safeguarding overall deliverability.

Troubleshooting: Clear attribution of deliverability problems to specific sending streams, allowing for faster diagnosis and resolution.

In practice, it’s rare for a shared subdomain setup to be truly seamless or sustainable in the long run. The dynamic nature of email deliverability, coupled with potential changes in ESP requirements or infrastructure, makes it a high-risk strategy. Prioritizing distinct subdomains for each ESP ensures a more stable, secure, and manageable email sending environment.

Views from the trenches

Best practices

Always use distinct subdomains for each email service provider to maintain isolated sender reputations and simplify DNS management.

Carefully review each ESP's requirements for DNS delegation and authentication to ensure compatibility and avoid conflicts.

Implement DMARC with a monitoring policy to gain full visibility into all sending sources for your domain and detect unauthorized use.

Regularly monitor your domain's health and deliverability across all ESPs, even with separate subdomains, to catch issues early.

Common pitfalls

Hitting the SPF 10-lookup limit when attempting to include multiple ESPs in a single SPF record for a shared subdomain.

One ESP inadvertently overwriting or conflicting with DNS records required by another ESP, leading to authentication failures.

Difficulty in pinpointing the source of deliverability issues or blocklist listings when multiple platforms share a sending identity.

Lack of a clear strategy for managing the Return-Path domain, which must be unique per ESP, even if the From domain is shared.

Expert tips

If a client insists on sharing a subdomain, thoroughly document all technical limitations and potential deliverability risks involved.

Ensure a robust monitoring strategy is in place to quickly detect any reputation or authentication problems across all sending streams.

Consider the long-term maintainability; managing complex shared DNS setups can become a significant operational burden over time.

Educate stakeholders on the importance of domain reputation isolation for consistent inbox placement and email program success.

Expert view

Expert from Email Geeks says: If your primary ESP controls the DNS for the From (5322.From) domain, sharing it with another ESP is often impossible because you won't be able to add the necessary additional DNS records required by the new platform.

Jan 4, 2023 - Email Geeks

Marketer view

Marketer from Email Geeks says: An ESP often wants to control the DNS for the Return-Path (5321.MAILFROM) domain to track bounces and manage feedback loops, making it difficult to share this specific domain across multiple providers.

Jan 4, 2023 - Email Geeks

Prioritizing isolated sending environments

While the prospect of sharing a subdomain between two different ESPs might appear to offer simplicity or cost savings, the technical complexities and significant risks to email deliverability and sender reputation far outweigh any perceived benefits. DNS conflicts, the inability to manage critical authentication records, and the entanglement of sender reputations can lead to severe deliverability issues, including emails consistently landing in spam or being outright blocked. You can learn more about why your emails are going to spam and how to fix it.

The recommended approach is to assign a unique, dedicated subdomain to each ESP. This strategy ensures proper DNS management, isolates sender reputation for each sending stream, and significantly simplifies troubleshooting. By doing so, you maintain maximum control over your email program, safeguard your brand's reputation, and optimize your chances of consistently reaching the inbox. Remember, a robust email strategy prioritizes long-term deliverability over short-term perceived efficiencies.

Ultimately, while the desire to simplify domain usage is understandable, the nuances of email authentication and reputation management dictate a more cautious and isolated approach. Investing in separate subdomains for each ESP is a foundational step toward achieving and maintaining excellent email deliverability.