Summary
DMARC bounces and SendGrid authentication issues are multifaceted and can arise from configuration errors, third-party issues, and malicious activity. Key causes include incorrect DKIM/SPF setup (DNS records, selector mismatches, alignment issues), problems with SendGrid (shared IPs, un-warmed IPs, SendGrid domains in headers), email forwarding, content modifications, rDNS misconfiguration, and DMARC policy application on subdomains. External factors such as list bombing can also trigger failures. Troubleshooting requires checking authentication headers, ensuring proper DNS settings, warming IPs, and maintaining domain alignment.
Key findings
- DKIM/SPF Configuration: Incorrect DNS records, selector mismatches in DKIM, and SPF configuration errors (missing includes, DNS lookup limits) cause DMARC failures.
- SendGrid Specific Issues: SendGrid can introduce issues like shared IPs (affecting reputation), un-warmed IPs (suspicious activity), and SendGrid domains appearing in authentication headers.
- DMARC Policy Application: Inappropriate or missing subdomain DMARC policies can cause authentication to fail.
- Alignment Problems: Failures occur when the 'From:' domain does not align with the authenticated domains (SPF and DKIM).
- Third-Party and Forwarding Issues: Email forwarding breaks SPF, and content modifications can invalidate DKIM signatures, triggering DMARC failures.
- External Attacks: List bombing triggers DMARC failures due to sudden changes in volume and sending patterns.
- rDNS Configuration: Incorrect reverse DNS (rDNS) configuration on SendGrid can affect deliverability and DMARC results.
Key considerations
- Examine Authentication Headers: Routinely inspect email headers to verify SPF and DKIM are functioning correctly and identify any issues with domain alignment or unexpected third-party domains.
- Ensure Correct DNS Settings: Verify that DMARC, SPF, and DKIM records are accurately configured in DNS, paying attention to includes, limits, and syntax errors.
- Warming Up IPs: When using dedicated IPs with SendGrid, gradually increase sending volume to establish a positive sending reputation.
- Proper Subdomain Configuration: Explicitly define DMARC policies for all subdomains to prevent inheritance issues and ensure authenticated sending.
- Monitoring DMARC Reports: Regularly analyze DMARC reports to identify failing authentication attempts, alignment issues, and possible spoofing attempts.
- Mitigation Strategies for Attacks: Implement rate limiting, stricter subscription verification, and other defenses to counteract list bombing attacks.
- Review rDNS Settings: Confirm correct reverse DNS settings for SendGrid dedicated IPs to ensure proper hostname verification.
What email marketers say
9 marketer opinions
DMARC bounces and SendGrid authentication issues can arise from a variety of misconfigurations and external factors. These include SendGrid domain issues in DKIM headers, problems related to shared IP addresses, inadequate IP warming, email forwarding, incorrect DNS settings, misconfigured reverse DNS, and DMARC 'reject' policies combined with failed authentication. Improper DMARC reporting configurations can also cause confusion. Overall, maintaining proper configurations, monitoring authentication, and handling IP reputation are key to preventing these issues.
Key opinions
- SendGrid DKIM Issues: SendGrid domains appearing in DKIM headers can lead to DMARC failures, requiring verification of email headers.
- Shared IP Reputation: Using SendGrid's shared IPs can result in DMARC failures if other users on the same IP send spam, affecting your sender reputation.
- Insufficient IP Warming: Failing to adequately warm up dedicated IPs on SendGrid can negatively impact reputation and cause DMARC rejections due to perceived suspicious activity.
- Email Forwarding: Email forwarding can break SPF authentication, leading to DMARC failures.
- Incorrect DNS Settings: Improperly configured DMARC, SPF, or DKIM records in DNS settings will cause authentication to fail.
- rDNS Configuration: Incorrectly configured reverse DNS (rDNS) on SendGrid can affect deliverability and cause DMARC failures.
- DMARC Reject Policies: Using a DMARC 'reject' policy without proper authentication setup can cause legitimate emails to bounce.
Key considerations
- Check Email Headers: Regularly check email headers to ensure correct DKIM and SPF configurations.
- Monitor IP Reputation: Be aware of your IP reputation, especially when using shared IPs, and take steps to maintain it.
- Warm Up Dedicated IPs: When using dedicated IPs, gradually warm them up to build a positive sending reputation.
- Proper DNS Configuration: Ensure that your DMARC, SPF, and DKIM records are correctly configured in your DNS settings.
- Monitor DMARC Reports: Regularly monitor DMARC reports to identify and address authentication issues.
- Verify rDNS Settings: For SendGrid users, confirm reverse DNS (rDNS) is properly configured for your dedicated IPs.
- Review DMARC Policies: Understand the implications of your DMARC policy (none, quarantine, reject) and adjust settings based on your authentication setup.
Marketer view
Email marketer from Email Marketing Tips shares that incorrect DNS settings can cause DMARC failures. If DMARC, SPF, or DKIM records are not correctly configured in your DNS settings, email authentication will fail. This can result in emails being rejected or marked as spam.
26 Oct 2021 - Email Marketing Tips
Marketer view
Email marketer from Mailhardener Blog explains that DMARC bounces can occur if you have a 'reject' policy and emails fail authentication. They also point out that incorrectly configured DMARC reporting can lead to confusion, as you might receive bounce notifications for legitimate emails that are being rejected due to DMARC policies at the recipient's end.
24 Jan 2024 - Mailhardener Blog
What the experts say
3 expert opinions
DMARC bounces and authentication issues can stem from several factors including issues specific to sending domains, improper subdomain handling with DMARC policies, and external attacks like list bombing. Sending domains may have authentication problems unique to their configuration or the receiving mail provider's policies. Furthermore, inheriting DMARC policies on subdomains without proper authentication setup causes failures. List bombing results in sudden volume increases and sending pattern changes, leading to authentication and deliverability issues.
Key opinions
- Domain Specificity: DMARC bounces can be specific to a particular sending domain or receiving mailbox provider.
- Subdomain Handling: Improper handling of subdomains with DMARC policies can lead to authentication failures.
- List Bombing: List bombing can trigger DMARC failures due to changes in sending patterns and increased volume.
Key considerations
- Domain Configuration: Ensure proper authentication setup for all sending domains to avoid domain-specific issues.
- Subdomain Policies: Explicitly define DMARC policies for subdomains to ensure proper authentication handling.
- Monitor for List Bombing: Implement measures to detect and mitigate list bombing attacks to prevent deliverability issues.
Expert view
Expert from Email Geeks shares that he would expect DMARC bounces to be specific to a particular sending domain or a particular receiving mailbox provider.
17 Apr 2024 - Email Geeks
Expert view
Expert from SpamResource explains that one cause of DMARC failure is improper handling of subdomains. If you have a DMARC policy set for your main domain, it also applies to subdomains unless you explicitly define a different policy for the subdomain. This can cause issues if the subdomains are not properly configured to send authenticated email.
9 Sep 2022 - SpamResource
What the documentation says
5 technical articles
DMARC bounces and SendGrid authentication issues often arise from incorrect DKIM and SPF configurations. Common causes include improperly configured DNS records, mismatched DKIM selectors, and issues with the signing process. SPF misconfigurations, such as missing 'include:' mechanisms or exceeding DNS lookup limits, can also lead to failures. Content modifications during email transit, breaking DKIM signatures, and misalignment between the 'From:' domain and SPF/DKIM authenticated domains are additional factors that contribute to these issues. Properly aligning SPF and DKIM is crucial for DMARC to pass.
Key findings
- DKIM Configuration Errors: Incorrect DKIM configuration, including DNS records and selector mismatches, is a major cause of authentication failures.
- SPF Misconfigurations: Improperly configured SPF records, such as missing includes or exceeding DNS lookup limits, can lead to DMARC failures.
- SPF/DKIM Alignment Issues: DMARC failures often stem from SPF and DKIM alignment problems, where the 'From:' domain does not match the authenticated domain.
- Content Modification: Email content modifications in transit, such as adding footers, can break DKIM signatures and cause DMARC to fail.
Key considerations
- Verify DKIM Records: Ensure DKIM DNS records are correctly configured with the correct selectors and signing process.
- Review SPF Records: Regularly review SPF records to ensure proper 'include:' mechanisms and adherence to DNS lookup limits.
- Maintain Domain Alignment: Ensure that the 'From:' domain aligns with the domains used for SPF and DKIM authentication.
- Prevent Content Modifications: Avoid modifying email content in transit to preserve DKIM signatures.
Technical article
Documentation from RFC explains that if the domain in the 'From:' header does not align with the domain used in the SPF or DKIM authentication, it will lead to a DMARC failure. DMARC requires alignment between the visible 'From:' domain and the authenticated domain.
10 Jun 2025 - RFC
Technical article
Documentation from Google Workspace explains that an improperly configured SPF record can lead to DMARC failures. Common misconfigurations include missing the 'include:' mechanism for third-party senders (like SendGrid), exceeding the DNS lookup limit, or having syntax errors in the SPF record.
31 Jul 2024 - Google Workspace
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