Why are emails being rejected with 'Domain of sender address does not exist' error?

Key findings

• DNS resolution failure: The core issue is that the receiving mail server cannot find or properly interpret the DNS records for the domain listed in the MAIL FROM command (also known as the envelope sender or return-path address).
• Internal network complications: This error frequently arises from issues within the recipient's internal network, such as tightly controlled DNS systems that do not correctly resolve external domains or subdomains, especially when delegated to third-party email service providers (ESPs).
• Misconfigured DNS records: Incorrect or missing MX, A, or PTR records for the sending domain can prevent proper domain validation by receiving servers. Sometimes, even if they exist, these records may not be publicly accessible or aligned with RFC standards. You can learn more about what DNS records are required.
• Recipient server filtering: The rejection originates from the recipient's mail server, which is performing inbound filtering. This implies that the problem lies with how their system is interpreting or resolving your domain's DNS, rather than an issue with your ESP's outbound sending (unless your ESP has an internal misconfiguration).

Key considerations

• Obtain full bounce messages and logs: Always request the complete bounce message and mail server logs from the recipient's IT team. These logs often contain more detailed information leading up to the 553 5.1.8 error, such as which hostname was being checked and the specific DNS error encountered.
• Verify public DNS: Ensure that your sending domain's DNS records (especially MX and A records) are correctly configured and publicly accessible. Tools like IONOS help articles often highlight the need for RFC-compliant DNS setup.
• Address internal DNS: If the issue is specific to emails sent to your own company or other organizations with restrictive internal networks, their IT department may need to adjust their internal DNS settings. This might involve duplicating external DNS entries internally or configuring their servers to correctly follow zone delegation. This is discussed in more detail in our guide on SMTP bounce reason 4.1.8.
• Check for authentication issues: While directly a DNS issue, sometimes underlying SPF, DKIM, or DMARC misconfigurations can indirectly contribute if the receiving server is stricter about domain validation during authentication checks. Ensure your authentication records are correctly set up, for example, by consulting a simple guide to DMARC, SPF, and DKIM.

Key opinions

• Frustration with diagnostics: Marketers frequently express bewilderment when emails to specific domains (like their own company's) fail, while general tests (e.g., to Gmail) pass. This inconsistency makes troubleshooting difficult without deeper technical insight.
• Reliance on IT/ESPs: They often depend heavily on their internal IT teams or email service providers to resolve these issues, highlighting a gap in direct control over the technical aspects of deliverability.
• Impact of internal networks: Many marketers find that corporate networks (their own or a recipient's) have unique DNS configurations or security policies that inadvertently block legitimate emails, leading to these types of rejections.
• Need for clear error messages: While bounce messages like Domain of sender address does not exist provide some insight, marketers often require more detailed logs from the recipient's server to pinpoint the exact cause.

Key considerations

• Collaborate with IT: Work closely with your internal IT team and the recipient's IT to share bounce messages and logs. Emphasize that the error is likely on the recipient's inbound mail server configuration, particularly related to DNS resolution within their network.
• Verify your DNS setup: Double-check your sending domain's DNS records (MX, A, SPF, DKIM, DMARC) for correctness and public accessibility. Issues with MX records can be temporary but significant.
• Test internal sending: If you're experiencing this issue when sending to your own company email addresses, it's a strong indicator of an internal DNS problem. Refer to our article on bounces due to invalid sender domain for further guidance.
• Document the problem: Keep detailed records of bounce messages, diagnostic attempts, and communications with IT teams to streamline the troubleshooting process.

Key opinions

• Internal DNS misconfiguration: Experts frequently identify internal corporate DNS configurations as the culprit, especially when these systems are tightly controlled and fail to correctly follow zone delegation for subdomains managed by external ESPs.
• Envelope sender validation: The error signals that the receiving mail server is performing a DNS lookup on the MAIL FROM (envelope sender) domain and failing to resolve it, a common behavior for older or stricter mail servers.
• Beyond ESPs: The consensus is that this error is typically a problem on the recipient's inbound MX cluster rather than an issue with the sender's ESP, given that external DNS checks often show the sender's configuration as healthy.
• Network perspective: Investigation must consider how the DNS resolution appears from the network perspective of the recipient's mail servers, as internal network policies or firewalls can block specific DNS queries.

Key considerations

• Request recipient logs: Advise clients to obtain detailed mail server logs from their recipient's IT team. These logs are crucial for identifying which hostname was queried and the specific DNS error response received.
• Diagnose internal DNS: Guide IT teams to examine their internal DNS resolvers. If they are not correctly querying external DNS servers or following zone delegation for the sending domain (especially for SPF DNS timeouts), they may need to implement internal DNS overrides or allow broader external queries.
• Verify full DNS chain: Ensure that all relevant DNS records (A, MX, PTR, SPF, DKIM) are correctly configured and propagated globally. Even seemingly healthy external setups can be problematic if internal DNS is misaligned, as discussed by Wordtothewise.
• Monitor DNS health: Encourage consistent monitoring of DNS health for the sending domain and subdomain, as highlighted by Spamresource, to quickly detect and address any resolution issues. For more on this, check out our guide on understanding email domain reputation.

Key findings

• RFC compliance: Documentation frequently stresses that the sender domain's DNS must be configured in accordance with relevant RFCs (Request for Comments) to be properly validated by receiving mail servers.
• Public DNS visibility: A common theme is the need for sender domains to have publicly available DNS records, even if authenticated within an ESP, to ensure global resolvability.
• MX record integrity: Errors can arise from incorrect MX records, which are essential for directing incoming mail, but also play a role in how sender domains are validated.
• Reverse DNS (PTR) records: Some documentation points to the absence or mismatch of Reverse DNS records (PTR records) for the sending mail server's IP address as a cause for such rejections, especially when a 550 error code is involved.

Key considerations

• Regular DNS checks: Documentation implicitly advises regular verification of all relevant DNS records for the sending domain to ensure they are consistently and correctly published.
• Consult DNS provider: If problems persist, documentation often recommends reaching out to your DNS provider or domain registrar for assistance in troubleshooting and correcting record configurations.
• Review ESP domain setup: Ensure that any authenticated domains within your ESP are correctly configured with all necessary DNS entries as required by the provider, including CNAME or TXT records for branding or authentication.
• Understand SMTP codes: Familiarize yourself with SMTP error messages, as they provide critical clues to the specific nature of the rejection. For instance, the 553 code generally indicates a permanent sender-related issue.