Why are there two domains in the 'Mail From' field of an email header, and how does it affect deliverability?

Key findings

• Domain separation: Emails inherently feature multiple domains beyond the visible 'From' address, including the envelope sender (or smtp.mailfrom), the HELO domain, and potentially others within authentication results.
• Authentication roles: SPF primarily authenticates the smtp.mailfrom domain and the HELO identity, while DKIM authenticates the domain specified in the d= tag of the signature.
• DMARC alignment: For an email to pass DMARC, either the SPF-authenticated smtp.mailfrom domain or the DKIM-signed domain must align with the header.from domain (the one recipients see). More on DMARC can be found in our guide to DMARC, SPF, and DKIM.
• Reputation tracking: ISPs consider the reputation of all domains involved in an email transaction, not solely those explicitly authenticated by SPF or DKIM. Domain credibility, history, and age are all factors that influence deliverability, as discussed by Audience Point.
• Authserv-id: In some Authentication-Results headers (e.g., from Microsoft), a second domain may appear as an authserv-id, which is the receiving server's identifier and typically irrelevant to the sending domain's reputation.

Key considerations

• Full header analysis: Always examine the full email headers to accurately distinguish between the various domains present and their specific roles in the email's journey and authentication process. Learn more about different terms for email From addresses.
• Subdomain strategy: Use separate subdomains for different email types, such as transactional, marketing, or business development emails, to isolate sender reputation and prevent issues with one stream from affecting another. We have further insights on why to use subdomains for email marketing deliverability.
• Authentication consistency: Ensure proper SPF and DKIM configuration for all sending domains and that they align with your header.from domain to pass DMARC and build a strong sender reputation.
• Sender reputation monitoring: Actively monitor the reputation of all domains you use for email sending, as a poor reputation on any involved domain can negatively impact deliverability.

Key opinions

• Deliverability hurdles: Many marketers encounter deliverability problems, especially with transactional emails, leading them to investigate unusual aspects of email headers, such as the presence of multiple domains.
• Reputation spillover: There's a strong concern that a poor reputation on one domain, particularly if it's also used for 'business development' or riskier email types, can contaminate the deliverability of other, more critical email streams, like transactional messages. This concern is also addressed in our page on how multiple domains affect sender reputation.
• Header opacity: The complexity and differing terminology within email headers often make it challenging for marketers to understand which domain is responsible for deliverability outcomes.

Key considerations

• Proactive header review: Regularly review raw email headers to identify all domains involved in the sending process and understand their roles, which can help troubleshoot issues like those described by EmailLabs.
• Reputation segmentation: Implement a strategy of using distinct subdomains for different email categories, especially separating high-volume or potentially riskier marketing emails from critical transactional messages, to protect overall domain reputation. This is a common strategy, also covered in our article on implications of using different root and subdomain email addresses.
• Targeted reputation management: Focus on improving the sender reputation of specific domains that are causing deliverability issues rather than assuming a blanket problem across all sending. Consider consulting a guide on recovering domain reputation.

Key opinions

• Envelope vs. header: The smtp.mailfrom is the true envelope sender (also known as return-path), which is what SPF primarily verifies, whereas header.from is the domain displayed to the recipient.
• SPF scope: SPF checks against both the HELO domain and the smtp.mailfrom domain, in addition to the sending IP address.
• ISP reputation factors: ISPs assess the reputation of all domains involved in an email's transmission, not just those explicitly covered by SPF or DKIM, to determine inbox placement. This means the reputation of domains in tracking links or image hosts could also affect deliverability, as discussed in our article about how image hosting affects deliverability.
• Microsoft's header quirks: Microsoft (Hotmail/Outlook) sometimes includes an authserv-id (which is typically the receiving domain) within Authentication-Results headers, which can be misinterpreted as a second sending domain. This is generally not factored into reputation. For more on this, consult Word to the Wise.
• RFC compliance: It is perfectly within RFC specifications to have multiple addresses in a 'From' header, though this is less common and can complicate DMARC alignment unless carefully managed. What RFC 5322 Says vs. What Actually Works can shed more light on this topic.

Key considerations

• Accurate header interpretation: Professionals should be adept at parsing raw email headers to correctly identify the purpose of each domain, distinguishing between sender identities, authentication results, and receiving server identifiers.
• Comprehensive reputation management: Focus on maintaining a strong reputation for all domains linked to your email sending activities, as ISPs (internet service providers) use a broad assessment of domain health. We cover similar factors in our guide to understanding your email domain reputation.
• ISP-specific nuances: Understand that different mailbox providers may handle and display email header information with slight variations, which can affect how domain information is interpreted.

Key findings

• Header importance: Email providers rely on header information to verify sender legitimacy and correctly route emails, as highlighted by EmailLabs.
• Domain alignment mandates: Failing to achieve domain alignment across the 'From', DKIM, and Return-Path headers can severely impair email deliverability, according to EmailLabs documentation.
• Subdomain benefits: Using separate subdomains for different types of emails (e.g., transactional vs. marketing) helps protect the reputation of the high-engagement email streams, as recommended by Mailmodo.
• Sender identification: Inbox providers use either the IP reputation or your domain reputation as the primary unique identifier for a sender, as noted by Customer.io.
• Return-Path importance: The 'Return-Path' email header is crucial for tracking bounces and managing email delivery, and customizing it can improve overall deliverability, according to Mailjet.

Key considerations

• Prioritize domain alignment: Ensure strict domain alignment for SPF, DKIM, and DMARC with your visible 'From' domain to satisfy mailbox provider requirements and improve inbox placement.
• Adopt subdomains strategically: Implement a subdomain strategy to separate email streams and protect the sender reputation of your primary domain. This is essential for managing different email campaigns effectively.
• Monitor authentication results: Regularly check authentication results for your emails to identify any issues related to domain inconsistencies or failures in SPF, DKIM, or DMARC. Tools like DMARC reports can provide valuable insights.