Is using different host name subdomains pointing to IPs a potential flag for email deliverability?

Key findings

• No inherent flag: Using various hostname subdomains pointing to different IPs is not automatically detrimental to email deliverability.
• Common practice: Shared IP pools routinely involve multiple domains utilizing the same IP addresses without issue.
• PTR records are crucial: Each sending IP must have a properly configured PTR (pointer) record that resolves back to its designated hostname.
• Purposeful use: Subdomains are often deployed to categorize email traffic, such as differentiating promotional from transactional emails, to better manage their respective reputations.

Key considerations

• DMARC alignment: Ensure that your DMARC alignment requirements are not overly strict, especially when using different subdomains in the From: header, to avoid accidental failures.
• Reputation management: Each subdomain effectively develops its own sender reputation, necessitating careful monitoring and consistent sending practices.
• Clear strategy: Always have a well-defined strategic reason for segmenting your email traffic with subdomains. An ill-conceived strategy can introduce unnecessary complexity and potential deliverability issues.
• DNS configuration for deliverability: It is critical for deliverability that the Return-Path domain has an SPF record that authenticates the source IPs. This is a fundamental requirement.

Key opinions

• Segmentation benefits: Marketers frequently use subdomains to separate distinct types of email traffic, like transactional versus marketing emails, to isolate any potential negative reputation impact.
• Reputation isolation: A primary reason for employing subdomains is their ability to shield the main domain's reputation, allowing separate subdomains to absorb the risk of deliverability issues.
• No automatic negative impact: Simply using a subdomain for sending does not inherently lead to worse deliverability compared to using the root domain.
• Strategic necessity: Subdomains are considered a valuable tool when there is a clear business justification for segmenting email sends.

Key considerations

• Warm-up new subdomains: New subdomains, even when sending from existing IPs, will require a proper warm-up process to establish their own reliable sending reputation.
• Consistency is key: Maintaining consistent sending volumes and fostering positive engagement for each subdomain is critical for achieving good deliverability and avoiding blocklists.
• Monitoring metrics: Closely monitor key metrics, such as open rates, click-through rates, and bounce rates for each subdomain, to detect and address issues promptly.
• Avoiding unnecessary complexity: Evaluate whether the deliverability benefits of using subdomains justify the added complexity in setup and ongoing management, as highlighted by industry practitioners.
• Protecting primary domain: Many marketers use subdomains to separate different email types to protect their primary domain's reputation.
• Subdomain impact: It is important to understand how using a subdomain for different email types affects your primary domain's reputation.

Key opinions

• Multiple domains on one IP is fine: Experts confirm that hosting multiple domains or subdomains on a single IP address is not problematic, citing shared IP pools as a common example.
• PTR record necessity: A properly configured PTR (pointer) record that maps back to the hostname is a fundamental requirement for EHLO commands during email transmission.
• DMARC alignment consideration: When deploying numerous subdomains, particularly in the From: header, it's wise to ensure DMARC alignment requirements are not overly strict to prevent accidental authentication failures.
• Strategic rationale: Experts emphasize the critical need for a clear, well-articulated strategic reason for using subdomains in the From: header, rather than simply adopting them based on assumptions.

Key considerations

• Avoid misinterpretations: Decisions regarding subdomain usage should be grounded in sound strategic planning, not misunderstandings of ESP advice or ill-informed consensus decisions.
• DNS configuration: Ensure that all DNS records, including SPF and DKIM, are accurately configured for each subdomain to facilitate proper email authentication. Learn more about how to ensure deliverability with different return-path addresses.
• Impact on reputation: While subdomains can help isolate reputation, poor sending practices on one can still indirectly affect the root domain or other related subdomains.
• Continuous learning: Even if an email strategy is already implemented, regularly re-evaluating and deepening the understanding of its implications can prevent future problems. This ongoing review is a staple of expert advice in the industry.

Key findings

• DNS independence: Subdomains maintain their own separate DNS entries, allowing them the flexibility to point to different IP addresses or host servers.
• Authentication requirements: Each subdomain utilized for sending email needs its own dedicated SPF and DKIM records to properly authenticate messages and prevent them from being flagged as spam.
• Return-path domain importance: The Return-Path domain (also known as the Bounce Address) must have a valid SPF record to effectively authenticate the source IP address.
• Domain reputation inheritance: Although subdomains can build their own individual reputations, a severely poor reputation or blocklisting of the root domain can still negatively affect its subdomains.

Key considerations

• PTR record alignment: The PTR record for the sending IP must correctly resolve to the hostname used in the EHLO/HELO command, which frequently involves a subdomain.
• DMARC policy application: DMARC policies are applied to the organizational domain, and subdomains can either inherit or have specific DMARC records configured, directly impacting alignment and validation. For more details, see the list of DMARC tags and their meanings.
• Complexity management: Introducing multiple subdomains escalates the complexity of DNS management and ongoing deliverability monitoring.
• Sender header implications: It's important to consider the impact of the Sender header and its associated domain on DMARC alignment and overall email deliverability, especially when using varied From: addresses. The basics of email subdomains provide further insight.