Summary
PowerMTA DKIM signing failures can stem from a wide array of issues encompassing configuration errors within PMTA, OpenDKIM, and virtual MTAs; problems related to DKIM keys and DNS records, including selector mismatches, key rotation oversights, and incorrect permissions; header-related issues like missing/malformed headers or exceeding header length limits; network-related problems such as firewall interference with DNS lookups; resource constraints on the server; and even modifications of the email body during transit. Underlying SPF failures can also complicate debugging. A systematic approach to verifying configuration, permissions, network settings, system resources, DNS records, and header integrity is essential to effectively diagnose and resolve these DKIM signing problems.
Key findings
- Configuration Problems: Incorrect settings in PMTA, OpenDKIM, or virtual MTAs, particularly concerning the 'domain' attribute, DKIM selectors, and signing configurations can cause failures.
- Key and DNS Issues: Mismatched DKIM selectors between PMTA and DNS, outdated DNS records after key rotation, or incorrect file permissions on the private DKIM key are frequent culprits.
- Header-Related Errors: Missing, malformed, or oversized email headers (especially From, To, Subject, and Date) can prevent successful DKIM signing.
- Network Interference: Firewall rules blocking DNS lookups can disrupt DKIM verification.
- Resource Constraints: Insufficient server resources (CPU, memory) under heavy load can lead to intermittent DKIM failures.
- Message Modification: Email body alterations during transit by clients or servers can invalidate the DKIM signature.
- Underlying SPF Issues: SPF failures may masquerade as DKIM failures, complicating troubleshooting.
- Clock Skew: Significant time differences between sending and receiving servers will cause validation failures.
Key considerations
- Configuration Review: Thoroughly examine PMTA, OpenDKIM, and virtual MTA configurations, paying close attention to DKIM-related settings.
- Key and DNS Verification: Ensure the DKIM selector matches in PMTA and DNS, that DNS records are updated post-key rotation, and that PMTA has proper access to the private key.
- Header Validation: Verify the presence and correct formatting of all essential email headers and adhere to header length limitations.
- Network Assessment: Check firewall rules to ensure they are not obstructing DNS lookups needed for DKIM verification.
- Resource Monitoring: Monitor server resources (CPU, memory) and optimize to avoid resource exhaustion during peak loads.
- SPF Evaluation: Investigate and address any underlying SPF failures that may be masking themselves as DKIM issues.
- Clock Synchronisation: Ensure all servers are synchronised to an accurate time source.
What email marketers say
11 marketer opinions
PowerMTA DKIM signing failures can arise from various configuration, permission, or environmental issues. These include incorrect DKIM DNS records, header issues (missing, malformed, or exceeding length limits), file permission problems with the private DKIM key, firewall interference with DNS lookups, insufficient system resources, and even modifications to the message body during transit. Configuration errors in PowerMTA itself or related tools (like OpenDKIM) are also potential culprits. Checking configuration, permissions, resource availability, and DNS settings is crucial for diagnosing and resolving DKIM signing issues.
Key opinions
- Configuration Errors: Incorrect DKIM DNS records or PMTA configuration settings (e.g., selector mismatch, missing domain attribute) are common causes of DKIM failures.
- Header Issues: Missing or malformed email headers (From, To, Subject, Date) or headers exceeding length limits can prevent successful DKIM signing.
- Permission Problems: Incorrect file permissions on the private DKIM key can prevent PMTA from accessing and using the key for signing.
- Network Interference: Firewall rules that block DNS lookups can interfere with DKIM verification processes.
- Resource Constraints: Insufficient system resources (CPU, memory) can lead to intermittent DKIM failures, especially under high server load.
- Message Modification: Some email clients or receiving mail servers may modify the message body during transit, invalidating the DKIM signature.
Key considerations
- DNS Verification: Verify the DKIM DNS records are correctly published and that the selector matches the one used in the PMTA configuration.
- Header Validation: Ensure all required headers (From, To, Subject, Date) are present and correctly formatted in the email messages.
- File Permissions: Confirm that the PMTA user has read access to the private DKIM key file.
- Firewall Configuration: Check for firewall rules that may be blocking DNS lookups required for DKIM verification.
- System Resources: Monitor system resources (CPU, memory) to ensure PowerMTA has sufficient resources for DKIM signing.
- Configuration Review: Review PMTA configuration files for syntax errors, misconfigurations, or incorrect virtual MTA settings.
Marketer view
Email marketer from Email Marketing Forum suggests that incorrect or missing DKIM DNS records are a common cause. Double-check that the DKIM record is published correctly in your DNS zone and that the selector matches the one used in your PMTA configuration.
22 Jun 2024 - Email Marketing Forum
Marketer view
Email marketer from StackOverflow shares that DKIM failures sometimes stem from exceeding header length limits. Long headers can be truncated during processing, invalidating the DKIM signature. Reducing header size may resolve the issue.
15 Sep 2024 - StackOverflow
What the experts say
3 expert opinions
DKIM signing failures in PowerMTA can be caused by several key issues. These include a mismatch between the DKIM selector in PowerMTA's configuration and the DNS record, failing to update the DNS record after a DKIM key rotation, and potentially underlying SPF failures that mask themselves as DKIM problems. Thoroughly verifying the DKIM selector, ensuring DNS records are up-to-date after key rotation, and checking SPF reports are crucial steps in troubleshooting these issues.
Key opinions
- Selector Mismatch: A mismatch between the DKIM selector configured in PowerMTA and the selector specified in the DNS record is a common cause of DKIM signing failures.
- Key Rotation Issues: Failure to update the DNS record with the new public key after a DKIM key rotation will cause signing to fail.
- Underlying SPF Failures: Some DKIM failures are actually due to underlying SPF failures, making debugging more difficult.
Key considerations
- Verify DKIM Selector: Ensure the DKIM selector configured in PowerMTA matches exactly the selector specified in the DNS record.
- Update DNS Records: After a DKIM key rotation, immediately update the DNS record with the new public key.
- Check SPF Reports: Review SPF reports to rule out SPF failures as a contributing factor to DKIM signing problems.
Expert view
Expert from Spam Resource notes that some DKIM failures are actually due to SPF failures, so debugging will be difficult without looking at the SPF reports. In addition you need to double check your DNS records.
7 Oct 2022 - Spam Resource
Expert view
Expert from Word to the Wise advises checking if the DKIM key has recently been rotated. If the DNS record hasn't been updated with the new public key, signing will fail.
4 Jun 2022 - Word to the Wise
What the documentation says
5 technical articles
DKIM signing failures in PowerMTA can stem from several configuration-related issues. Incorrectly configured 'domain' attributes in the `<dkim>` block, missing or malformed required headers (From, To, Subject, Date), clock skew between servers, errors in OpenDKIM configuration (which PMTA uses for signing), and incorrectly configured virtual MTAs can all lead to these failures. Thoroughly reviewing PMTA, OpenDKIM, and virtual MTA configurations, ensuring header validity, and synchronizing server clocks are critical steps in diagnosing and resolving these problems.
Key findings
- Domain Attribute Errors: Incorrectly configured 'domain' attribute in the `<dkim>` block of the PMTA configuration file can cause DKIM signing failures.
- Missing/Malformed Headers: Missing or malformed required headers (From, To, Subject, Date) will lead to signing failures.
- Clock Skew: Clock skew between the signing and verifying servers can result in DKIM signature verification failures.
- OpenDKIM Configuration: Configuration errors in OpenDKIM, which PMTA utilizes for signing, directly affect PMTA's signing capabilities.
- Virtual MTA Configuration: Incorrectly configured virtual MTAs can result in DKIM failures; each virtual MTA needs its own signing domain, selector, and private key.
Key considerations
- Check Domain Attribute: Ensure the 'domain' attribute in the `<dkim>` block matches the 'From' address domain.
- Validate Headers: Verify that all required headers (From, To, Subject, Date) are present and correctly formatted in the email messages.
- Synchronize Clocks: Synchronize clocks between the signing and verifying servers to minimize clock skew.
- Review OpenDKIM Logs: Check OpenDKIM logs for errors that might be affecting PMTA's signing capability.
- Verify Virtual MTA Setup: Confirm that each virtual MTA is properly configured with its own signing domain, selector, and private key.
Technical article
Documentation from PMTA User Guide explains that incorrectly configured virtual MTAs can result in DKIM failures. Confirm that each virtual MTA is properly configured with its own signing domain, selector and private key.
7 Apr 2024 - PMTA User Guide
Technical article
Documentation from RFC Editor explains that DKIM signature verification failures can result from clock skew between the signing and verifying servers. If the timestamp in the DKIM signature is too far in the past or future, verification may fail.
24 Apr 2022 - RFC Editor
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