Summary
A sudden decline in DKIM signature authentication indicates a significant deliverability issue, often stemming from recent, unintended changes across your email infrastructure. Diagnosing such a drop involves systematically inspecting various components, from DNS records to mail server configurations and the full email sending path. DMARC reports are a crucial starting point for pinpointing the exact nature of these authentication failures.
Key findings
- DNS record integrity: A primary cause is issues with the DKIM DNS TXT record itself. This could involve the record being missing, incorrectly formatted, having an incorrect hostname (selector), or being inadvertently modified or removed during DNS changes or migrations. Propagation delays due to DNS caching can also create a perceived drop.
- Mail server configuration changes: Updates to Mail Transfer Agent (MTA) software (e.g., Postfix, Exim), alterations to opendkim or dkimproxy settings, or the deployment of a new server without proper DKIM setup can break signing. Using an incorrect private key or having unconfigured sending IPs also leads to failures.
- Key management issues: DKIM keys have a lifecycle; their expiration or improper rotation can lead to sudden signature drops.
- Email service provider (ESP) actions: Sometimes, the ESP might unintentionally break DKIM signing for your domain or alter the sender from your primary domain to an ID-specific subdomain, invalidating the signature.
- Intermediate device interference: New security appliances, firewalls, or email gateways can modify email headers or content, even subtly, after the DKIM signature has been applied, rendering it invalid.
- System updates and time synchronization: Operating system or mail server software updates might unintentionally alter configurations. In rare instances, significant discrepancies in system time or timezones between signing and verifying servers can contribute to validation problems.
Key considerations
- Analyze DMARC reports: DMARC reports provide essential details on which messages failed DKIM authentication and their specific origins, making them an invaluable tool for initial diagnosis.
- Verify DNS records: Confirm the presence, correct format, and matching selector of your DKIM TXT record. Check for any recent DNS changes that might have impacted it, and be mindful of DNS propagation delays.
- Audit mail server configurations: After any system or software updates, or when deploying new servers, verify that your MTA and DKIM signing libraries (like OpenDKIM) are correctly configured for your domains and are using the proper private keys.
- Inspect outbound mail flow: Determine if mail is routing through an unexpected server or service that lacks DKIM signing capabilities, or if any intermediate security solutions are modifying message headers after signing.
- Manage DKIM keys proactively: Ensure a robust process for DKIM key rotation and renewal to prevent issues arising from expired keys.
- Configure all sending sources: If you use multiple IP addresses or domains for sending, confirm that DKIM is properly configured for every one of them to avoid perceived drops when traffic shifts.
- Maintain accurate system time: While less common, ensuring accurate time synchronization across your mail servers can prevent subtle authentication issues.
What email marketers say
8 marketer opinions
A sudden drop in DKIM signature validity often signals recent, perhaps unnoticed, modifications within your email sending environment. Diagnosing this issue requires a systematic investigation of various components, ranging from your email service provider’s settings and mail server configurations to network security devices and DNS record propagation. DMARC reports remain an indispensable tool for initially pinpointing the exact nature and origin of these authentication failures, providing crucial insights into what might be amiss.
Key opinions
- ESP configuration changes: Your email service provider might have unexpectedly altered the sender from your primary domain to an ID-specific subdomain, which invalidates the DKIM signature. This is a common cause reported by users.
- Mail server configuration issues: Recent changes to your mail server setup, including updates to MTA software or alterations in opendkim/dkimproxy configurations, can reset or modify DKIM signing parameters. Also, deploying a new server without proper DKIM setup is a frequent cause.
- Intermediate device interference: New or reconfigured security appliances, firewalls, or email gateways in your mail flow path can modify email headers or content. Even subtle alterations can invalidate a DKIM signature post-signing.
- Unconfigured new sending sources: If new sending IP addresses or mail servers were recently added or migrated, and their DKIM configuration was overlooked, it can lead to a sudden drop in validated signatures from those sources.
- DNS caching and propagation delays: DNS caching issues can delay the global propagation of correct DKIM records following updates. This means some validating servers may still attempt to use outdated, incorrect, or non-existent records, leading to validation failures.
- System and software update impacts: Updates to operating systems, mail server software, or underlying libraries might unintentionally alter configurations or dependencies essential for DKIM signing, causing a sudden drop in authenticated emails.
- Firewall or network policy interference: New or modified firewall rules or network security policies can occasionally interfere with outbound email, potentially by altering the email stream or blocking critical ports/services, which can lead to emails being sent without a proper DKIM signature.
- System time discrepancies: Though rare, significant differences in system time or timezones between the signing server and the verifying server can, in some edge cases, contribute to DKIM signature validation issues.
Key considerations
- Review your email service provider settings: If using an ESP, confirm that no unexpected changes have occurred, such as your sender domain being switched to an ID-specific subdomain, which can inadvertently invalidate your DKIM signature.
- Audit mail server and MTA configurations: Thoroughly check your mail server setup, including your Mail Transfer Agent (MTA) software (like Postfix or Exim) and any DKIM-specific configurations (such as opendkim or dkimproxy). Look for any recent updates or changes that might have altered signing parameters or reset settings.
- Inspect intermediate network devices: Investigate whether any new security appliances, firewalls, or email gateways have been introduced or reconfigured. These devices can subtly modify email headers or content after the DKIM signature is applied, rendering it invalid.
- Verify all sending IP addresses and servers: Ensure that every outbound mail server and IP address in your infrastructure is properly configured for DKIM signing. Overlooking the DKIM setup for newly added or migrated servers can lead to a sudden drop in valid signatures.
- Check for DNS propagation delays: If your DKIM DNS records were recently updated, be mindful of TTL (Time To Live) settings and potential DNS caching issues. It can take time for changes to propagate globally, meaning some recipients might still try to validate against old or incorrect records.
- Review recent system and software updates: Examine logs for any recent operating system, mail server software, or related library updates. These updates might unintentionally alter configurations or dependencies necessary for correct DKIM operation.
- Ensure accurate system time synchronization: While uncommon, significant discrepancies in system time or timezones between your signing server and the verifying server can contribute to DKIM validation issues. Maintaining accurate time synchronization is a good general practice.
Marketer view
Marketer from Email Geeks shares that he investigated his DMARC reports and discovered his email service provider, Brevo, unexpectedly changed the sender from his primary domain to an ID-specific subdomain. This change was the cause of his DKIM signature invalidation.
2 Mar 2024 - Email Geeks
Marketer view
Email marketer from Server Fault shares that a common cause for a sudden DKIM signature drop is recent changes to the mail server configuration. This could involve: * An update to the MTA (Mail Transfer Agent) software, like Postfix or Exim, which might have reset or altered DKIM signing parameters. * Changes to opendkim or dkimproxy configurations. * Deployment of a new server without proper DKIM setup.
17 Jun 2022 - Server Fault
What the experts say
2 expert opinions
A sudden drop in DKIM signature authentication rates demands immediate attention to maintain email deliverability. This common issue is usually a symptom of recent alterations within your email infrastructure. DMARC reports are instrumental for initial diagnosis, offering granular data on which messages are failing and their specific origin. Effective troubleshooting involves a thorough examination of DNS records, mail server configurations, and the entire outbound mail path to pinpoint the exact cause of signature invalidation.
Key opinions
- Dmarc report insights: DMARC aggregate and forensic reports offer crucial details on DKIM authentication failures, including the specific messages and their sending origins, which can directly point to the source of the drop.
- DNS record mismatches: Issues often stem from the DKIM DNS TXT record itself. This includes typographical errors in the public key, the selector in the DNS record not matching what the mail server is using, or the record being missing or incorrectly published.
- Mail server configuration state: Problems can arise if the DKIM signing library on the mail server is not properly configured or active. This includes ensuring the server is explicitly set to sign mail for your specific domain and that the correct private key is being used for signing.
- Outbound mail flow diversions: Emails might unexpectedly route through a different server or service that is not configured for DKIM signing. Additionally, an intermediate step in the mail stream (like a proxy or security appliance) could be modifying messages after signing, thereby invalidating the signature.
- Email service provider (esp) actions: In some cases, your email service provider might have inadvertently broken DKIM signing for your domain, or made changes that affect how your outgoing mail is signed, leading to a sudden drop in valid signatures.
Key considerations
- Analyze dmarc reports thoroughly: Begin your diagnosis by examining your DMARC reports. These reports provide invaluable insight into which messages failed DKIM authentication and from what source, significantly narrowing down the scope of your investigation.
- Verify dkim dns records: Double-check your domain's DNS TXT record for the public key. Confirm that the selector used in the DNS record precisely matches the selector your mail server is employing. Look for any typographical errors or ensure the record has not been accidentally deleted or altered.
- Review mail server configuration: Inspect your mail server's DKIM signing library configuration. Confirm that it is correctly set up, active, and configured to sign mail for your specific domain, using the appropriate private key.
- Map outbound mail flow: Trace the complete path of your outbound email. Identify if any emails are being routed through a server or service not properly configured for DKIM signing, or if any intermediate network devices are modifying the email headers or content post-signing.
- Communicate with your esp: If you utilize an email service provider, reach out to them to inquire about any recent changes on their end that might have affected DKIM signing for your domain.
Expert view
Expert from Email Geeks explains that DMARC reports are incredibly helpful for diagnosing issues like DKIM signature drops, as they provide details on what messages failed and their origin. She suggests that such a drop could indicate that the email service provider has broken DKIM signing for the domain.
21 Jun 2024 - Email Geeks
Expert view
Expert from Word to the Wise explains that diagnosing a sudden drop in DKIM signatures often involves checking several key areas. These include: - verifying the DNS record for your public key, ensuring it is published correctly and that the selector in your DNS record matches what your mail server is using (typographical errors or missing records are common causes); - examining your mail server's configuration, confirming that the DKIM signing library is properly configured and active, which includes checking if the server is set to sign mail for your specific domain and if the correct private key is being used; - confirming the outbound mail flow, as sometimes mail may be routing through a different server or service that is not configured for DKIM signing, or an intermediate step in the mail stream might be modifying messages in a way that invalidates the signature.
24 Mar 2023 - Word to the Wise
What the documentation says
6 technical articles
A sudden decrease in successful DKIM authentication for outgoing emails often signals specific, addressable issues within your sending infrastructure, affecting deliverability. While DMARC reports remain invaluable for identifying the scope of such failures, pinpointing the exact cause requires a focused investigation. Common culprits include misconfigurations in DNS records, unmanaged key lifecycles, or unexpected alterations to email headers during transit. Furthermore, inconsistent DKIM setups across all active sending domains or subdomains can lead to a deceptive decline in overall authentication rates.
Key findings
- DNS record integrity and type: Problems with the DKIM DNS TXT record are a leading cause. This includes the record being absent, improperly formatted, having a mismatched hostname (selector), or being inadvertently modified or removed during DNS changes. Additionally, using CNAME records that point to incorrect or non-existent targets, or a broken CNAME chain, can also cause issues.
- Key expiration and rotation: DKIM keys possess a finite validity period. A sudden drop in signatures can directly result from an expired key or a failure in the key rotation process, meaning the public key in DNS no longer matches the private key used for signing.
- Incorrect DKIM selector usage: A specific type of DNS record issue, an incorrect selector means the identifier used by the sending mail server does not match the selector in the published DNS record. This prevents recipient servers from locating the correct public key for validation.
- Unintended header modification: Some mail servers or security solutions may add, remove, or alter certain email headers (for instance, 'Received,' 'Subject,' or 'Date') after the DKIM signature has been applied. This post-signing modification invalidates the signature, causing authentication to fail.
- Multi-domain/subdomain configuration gaps: If an organization sends email from multiple domains or subdomains, and DKIM is only configured for a subset of them, a shift in sending volume to an unconfigured domain can cause a perceived overall drop in DKIM signatures. Each sending domain or subdomain requires its own validated DKIM setup.
Key considerations
- Verify DNS records for accuracy: Scrupulously check the DKIM TXT record in your DNS. Ensure it is present, correctly formatted, and that the selector string matches what your mail server is using. Also, investigate if any CNAME records are pointing to valid and active targets, and that no inadvertent changes to record types have occurred.
- Manage DKIM key lifecycles: Implement a robust process for managing your DKIM keys, including regular renewal and rotation. Proactively check the validity period of your current keys to prevent expiration from causing sudden authentication failures.
- Confirm DKIM selector consistency: Make certain that the DKIM selector specified in your DNS record precisely matches the selector configured on your sending mail server. A mismatch will invariably lead to authentication failure as recipient servers cannot find the corresponding public key.
- Inspect email header integrity: Analyze outgoing email headers to identify any modifications occurring after DKIM signing. Look for intermediate mail servers, security solutions, or other processes that might be altering headers like 'Received' or 'Subject,' as this invalidates the signature.
- Ensure comprehensive DKIM configuration: For organizations sending from multiple domains or subdomains, verify that DKIM is properly set up and validated for each sending identity. A gap in configuration for even one active sending source can significantly impact your overall DKIM authentication rates.
Technical article
Documentation from Cloudflare Docs explains that a sudden drop in DKIM signature often points to an issue with the DNS record itself. Key points include: * The DKIM TXT record might be missing, incorrectly formatted, or have an incorrect hostname (selector). * DNS changes or migrations can inadvertently remove or modify the DKIM record, leading to authentication failures. Verifying the DNS record is the first step.
15 Jun 2022 - Cloudflare Docs
Technical article
Documentation from SendGrid Documentation explains that DKIM keys have a lifecycle, and a sudden drop in signature could be due to key expiration or improper key rotation. It's crucial to ensure your DKIM keys are renewed or rotated as per your policy to avoid authentication failures. Checking the validity period of the current key and the process for key management is a vital diagnostic step.
2 Apr 2022 - SendGrid Documentation
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