What DNS records are required to solve '450 4.1.8 Sender address rejected: Domain not found' errors?

Key findings

• DNS resolution: The core issue is the recipient's mail server failing to find the sender's domain's DNS records, particularly A, AAAA (IPv6), or MX records.
• Affected domains: This error typically refers to the RFC5321.From (envelope sender) domain or the RFC5322.From (header From) domain.
• Missing records: Even if MX records exist for a domain, the absence of corresponding A or AAAA records can trigger this error. This suggests some receiving systems require both for proper validation.
• DNSSEC impact: An insecure DNSSEC status or misconfiguration can also lead to domain not found errors, as it affects the integrity and authenticity of DNS data. For more details, consider reading about best practices for DNS lookups.
• ESP configuration: While your ESP might handle much of the DNS, ensuring the domains you use for sending (e.g., your From domain) have appropriate A, AAAA, and MX records is crucial. This is particularly relevant when setting up email subdomains.

Key considerations

• Comprehensive DNS checks: Perform thorough DNS lookups for all relevant domains (e.g., ml.hostname.com and mtaX.ml.hostname.com) using tools like DNSViz to verify A, AAAA, MX, and DNSSEC status.
• RFC compliance: Ensure DNS records align with RFC standards, particularly for RFC5321 (envelope sender) and RFC5322 (header From) domains, as some receivers strictly enforce these.
• DNS record presence: If a domain is used in the From address, it generally needs to have either an A record or an MX record to be resolvable. Ideally, both should be present if the domain is intended to receive mail.
• DNSSEC enablement: If your DNS provider supports it, enabling DNSSEC can add a layer of security and prevent spoofing, potentially helping with domain validation at the recipient's end. Consult your DNS host's documentation, like SendGrid's recommendations, for specific instructions on domain authentication records.

Key opinions

• DNS completeness: Marketers often find that simply having MX records is not enough; some systems also require A or AAAA records for the sending domain to resolve correctly.
• Subdomain complexity: The use of multiple dedicated IPs and rotating subdomains (e.g., mtaX.ml.hostname.com) for sending, while the From address uses a higher-level domain (e.g., ml.hostname.com), can lead to unexpected DNS resolution failures. Learn more about DNS records for email sending subdomains.
• Debugging approach: Many suggest a systematic approach to debugging, including checking all DNS records for the RFC5321.From and RFC5322.From domains.
• DNSSEC oversight: Lack of DNSSEC enablement or an insecure status for the SOA record can also contribute to these errors, indicating a need for better DNS security.

Key considerations

• Verify all sending domains: Don't just check the main domain. Ensure all subdomains or specific hosts mentioned in the email headers (e.g., Received headers) have correct DNS configurations. This is important for address rejected errors.
• DNSSEC status: Actively check if DNSSEC is enabled and configured correctly for your domain. If it's showing as insecure, that needs to be addressed.
• A and AAAA records: Even if a domain is not primarily used for web hosting, consider adding an A record (and AAAA if applicable) for the From domain to ensure broadest compatibility with receiving servers. Some systems expect a domain that sends mail to also be resolvable via an A record.
• ISP-specific requirements: Be aware that some ISPs or security solutions (like Proofpoint) might have stricter or unique requirements for DNS resolution. Regularly check their postmaster pages for specific guidance, such as SpamTitan's common triggers.

Key opinions

• Direct access needed: Experts agree that precise diagnosis of 'Domain not found' errors is nearly impossible without direct access to and examination of the affected domain's DNS records. For troubleshooting, it's often best to troubleshoot intermittent email delivery failures.
• RFC distinctions: Understanding the difference between RFC5321 (envelope sender) and RFC5322 (header From) domain records is critical, as the 'Domain not found' error can relate to either.
• A/MX requirements: The error commonly stems from a lack of A, AAAA, or MX records for the RFC5321 From domain, implying that the sending domain cannot receive mail back.
• DNSSEC importance: DNSSEC issues, such as an insecure status or misconfiguration, are identified as potential contributors to these domain resolution failures, impacting trust and deliverability. This also relates to PTR records and their essential role.

Key considerations

• Systematic DNS lookups: A systematic approach involving multiple DNS lookups (A and MX records) for both RFC5321 and RFC5322 domains is the recommended strategy for diagnosing. This includes verifying that authoritative servers are correctly serving these records.
• Inbound mail flow: The underlying problem is often that the sending domain cannot receive mail back, which triggers the 'Domain not found' rejection. Ensuring the domain has proper MX records for inbound mail is crucial.
• Tools for diagnosis: Utilize advanced DNS diagnostic tools, such as DNSViz, to visualize DNS configurations and identify potential delegation issues or insecure statuses, particularly related to DNSSEC.
• IPv6 consideration: When checking for A records, also include AAAA records, as some receiving systems may use IPv6 for resolution. The absence of AAAA records could contribute to the 'Domain not found' error if the recipient system is attempting IPv6 lookups.

Key findings

• RFC 5321 (SMTP): Specifies the protocol for email transfer and assumes the envelope sender domain is resolvable for bounce messages, implicitly requiring A, AAAA, or MX records. See more about RFC 5322 vs what actually works.
• RFC 5322 (Internet Message Format): Defines the format of email messages, including the From header, for which the domain also needs to be resolvable to ensure legitimate communication.
• DNSSEC standards: DNS Security Extensions (DNSSEC) provide authentication of DNS data, mitigating spoofing and other attacks. An insecure DNSSEC status can undermine domain trust and lead to validation failures by receiving servers.
• Domain authentication records: Documentation from major email service providers (ESPs) and security vendors frequently lists A, AAAA, and MX records as essential for sender domain authentication and to prevent 'Domain not found' rejections. These are often prerequisites even before SPF, DKIM, and DMARC checks. Read more about a simple guide to DMARC, SPF, and DKIM.

Key considerations

• Universal domain resolvability: The sender domain, whether in the envelope or header, must be universally resolvable across the internet via standard DNS lookups (A, AAAA, MX records). This includes ensuring proper delegation from the top-level domain.
• MX record for replies: Even if a domain is used solely for sending, it must have a valid MX record pointing to a mail server that can accept inbound mail (even if silently discarded) to prevent rejections based on the assumption it cannot receive bounces or replies.
• A and AAAA records for source: For sender domains (particularly the RFC5321.From or HELO domain), the presence of A and/or AAAA records is critical for receivers to perform forward DNS lookups and establish trust.
• DNS hosting reliability: Use a reliable DNS hosting provider and ensure DNSSEC is correctly configured and enabled. If Cloudflare is used for DNS, ensure DNSSEC is activated in their settings to secure your domain's DNS information.