Why is Power MTA failing to sign DKIM for some outbound emails?
Summary
What email marketers say11Marketer opinions
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.
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.
Email marketer from ServerFault.com explains that PowerMTA requires enough system resources to properly sign messages. High server load can cause intermittent DKIM failures.
Email marketer from TechnicalForums.net shares that the PMTA configuration files might have syntax errors or misconfigurations that cause DKIM signing to fail intermittently. Ensure that all configuration parameters are correctly set according to the PMTA documentation.
Marketer from Email Geeks explains that the failure can happen if the email goes out via {default} and general signing is not set up, or if a header (possibly reply-to) is missing.
Email marketer from Reddit mentions that incorrect file permissions on the private DKIM key can prevent PMTA from accessing it. Ensure the PMTA user has read access to the key file.
Email marketer from Mailop.org suggests checking if any firewall rules are interfering with the DNS lookups required for DKIM verification. A misconfigured firewall can block access to DNS servers.
Email marketer from Email Deliverability Blog emphasizes the importance of including all required headers, such as 'Date' and 'From', in the email. PMTA may fail to sign if these headers are missing.
Email marketer from Email Security Blog shares that some email clients or receiving mail servers may modify the message body during transit, invalidating the DKIM signature. This can lead to intermittent failures.
Email marketer from LinuxAdminForums explains that PMTA must have the correct user and group permissions to read the DKIM key file. If PMTA can't access the key, it won't be able to sign outbound messages.
Marketer from Email Geeks asks for clarification, questioning whether the issue is with adding the DKIM domain in the config file or with DKIM-signing emails despite the DKIM domain being in the config file.
What the experts say3Expert opinions
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.
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.
Expert from Word to the Wise explains that a common cause is a mismatch between the DKIM selector configured in PowerMTA and the selector specified in the DNS record. Ensure they match exactly.
What the documentation says5Technical articles
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.
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.
Documentation from PowerMTA.com explains that DKIM signing failures can occur if the 'domain' attribute is not correctly configured in the `<dkim>` block of the PMTA configuration file. Ensure the domain matches the 'From' address domain.
Documentation from OpenDKIM.org explains that PMTA uses OpenDKIM to sign messages, configuration errors in OpenDKIM directly affect PMTA's signing capability. Check OpenDKIM logs for errors.
Documentation from PowerMTA.com states that PowerMTA requires specific headers to be present in the email for successful DKIM signing. Missing or malformed headers, especially 'From', 'To', 'Subject', and 'Date', can cause signing failures.