Can email signatures, especially via Exclaimer, cause SPF or DKIM failures and impact email delivery?

Key findings

• DKIM re-application: Services like Exclaimer often strip the original DKIM signature and rely on Microsoft 365 to re-apply it after processing, which should generally maintain authentication. Issues with Microsoft 365 DKIM re-signing could still arise.
• Content modification impact: Exclaimer can add significant content, such as base64 encoded images and links to its own (un-whitelabeled) domains. This can increase email size and complexity, potentially triggering spam filters.
• Authentication status vs. deliverability: An email can pass SPF and DKIM but still face deliverability issues if other factors, like content, sender reputation, or recipient policies, flag it as suspicious. This is a common deliverability challenge, even when authentication is technically passing.
• DMARC policy implications: A strict DMARC policy of p=reject can lead to emails being discarded if any part of the processing chain inadvertently causes authentication to fail, even temporarily. Conversely, leaving DMARC in p=none means no action is taken on failed messages, which reduces protection but can prevent accidental rejections. You can learn more about DMARC record and policy examples here.
• Root cause shift: Initial suspicions about signature-related SPF/DKIM failures may lead to discovering other, more critical issues, such as a domain being listed on an IOC (Indicators of Compromise) blocklist or blacklist, which has a direct and severe impact on deliverability.

Key considerations

• Verify authentication post-processing: Always check email headers after they have passed through any signature service to confirm SPF, DKIM, and DMARC are still passing and aligned. This is the only way to confirm the technical state of the email as it leaves your infrastructure.
• Examine rejection messages: If emails are rejected, the rejection message provides critical clues about the specific reason for non-delivery. This data is indispensable for accurate troubleshooting.
• Investigate content filtering: If authentication passes but emails go to spam, examine the content added by the signature service, especially embedded images or links, as these can trigger content-based spam filters or specific IT policies blocking certain types of content (e.g., base64 encoded). More on best practices for email signature security is available.
• Assess domain reputation: Ensure your domain is not listed on any blacklists or blocklists. A compromised domain or one listed due to suspicious activity will severely impact deliverability regardless of signature setup. Our guide on email blacklists can help.
• Holistic view: Consider all changes around the time deliverability issues began. Often, the problem isn't a single factor but a combination of issues or recent modifications to your email infrastructure or policies.

Key opinions

• Signature services altering emails: Some marketers suspect email signature services of rewriting messages in ways that could potentially interfere with DKIM signatures, necessitating proper re-application by the sending server (e.g., Microsoft 365) after the signature has been added.
• Deliverability impact from Exclaimer: There are reports from some users of emails relayed through Exclaimer consistently going to junk folders on platforms like Office 365, even when the domain health and content are otherwise good. This issue sometimes resolves upon disabling Exclaimer or switching to an alternative service.
• Content additions as a flag: The addition of large base64 encoded images and un-whitelabeled links by signature services is seen by some as a potential flag for enterprise spam filters, contributing to deliverability challenges.
• Importance of proper authentication: Even with signature services, ensuring correct SPF, DKIM, and DMARC setup remains a top priority for improving deliverability and avoiding spam folders.
• Broader issues: When deliverability problems extend to both sending and receiving emails, it often points to a systemic issue beyond just an outgoing email signature service, such as a domain being on a blocklist or other infrastructure changes.

Key considerations

• Demand specific data: Before blaming email signatures, IT teams should provide concrete data, such as rejection messages, sender logs, or specific blocklist listings, to diagnose deliverability issues accurately.
• Monitor email flow: Use tools to analyze email headers after they pass through any signature service to confirm SPF, DKIM, and DMARC are correctly aligned and passing, especially when using a DMARC p=reject policy.
• Review recent changes: If deliverability issues suddenly appear, investigate any recent changes made to email infrastructure, DMARC policies, or signature configurations around that time. You can also explore why authentication failures occur despite correct setup.
• Consider content impact: While email authentication is key, the actual content of the email, including what signature services add, can influence deliverability. For instance, emails that include signatures can sometimes face delivery issues.

Key opinions

• Mail forwarding breaking signatures: An expert from Email Geeks explained that different types of signatures exist, and mail being forwarded (or re-routed through a service) can indeed break existing DKIM signatures, leading to failures if not properly handled. Learn more about how email forwarding affects SPF, DKIM, and DMARC validation.
• Exclaimer and DKIM re-application: An expert from Email Geeks confirmed that Exclaimer rewrites the message, so the DKIM signature must be applied *after* the message passes through Exclaimer, typically by Microsoft 365.
• Risk of p=reject with intermediaries: If Exclaimer (or similar services) strips DKIM before forwarding, and the domain publishes a DMARC policy of p=reject, it instructs recipient servers to discard emails where authentication has been removed or invalidated.
• Content's role in filtering: An expert noted that Exclaimer adds a significant amount of data, including base64 encoded images and links to its own (un-whitelabeled) domain. While not a direct authentication failure, this content can make an email borderline for enterprise filters.
• Data is paramount: Multiple experts emphasize that diagnosing deliverability issues without detailed data, such as actual rejection messages or specifics about spam placement, is nearly impossible and leads to speculation.

Key considerations

• Verify authentication reports: If a tool reports that your mail is correctly signed and aligned (passing SPF and DKIM), then the problem is likely not a direct SPF or DKIM failure caused by the signature service. You may need to review DMARC reports from Google and Yahoo.
• Look for underlying issues: If deliverability problems include both sending and receiving, it strongly suggests a broader network or domain issue, rather than a problem solely with an outgoing email signature service.
• Address DMARC policy carefully: If DMARC is set to p=reject without proper reporting or monitoring, it can hide problems where legitimate emails are being discarded due to unexpected authentication failures from intermediate services. Consider how to safely transition DMARC policy to quarantine or reject.
• Ask critical questions: When presented with deliverability issues, the first question should always be, "What changed?" around the time the problems began, as this often reveals the root cause.

Key findings

• DKIM re-signing process: Documentation for signature services often states that they strip the original DKIM signature upon receiving an email but expect the mail server (e.g., Microsoft 365) to re-apply it after the signature has been processed, before the email is sent to the recipient.
• SPF and regional records: Some signature services, like Exclaimer, have regional SPF records. It is crucial to ensure that messages originating from the specific region are correctly passing SPF validation and that your domain's SPF record includes all necessary authorized sending sources.
• DKIM signing with root domain: Official guidelines, particularly for Microsoft 365, recommend ensuring that DKIM signing is configured with the root domain, not the initial *.OnMicrosoft.com setup domain, to ensure proper alignment and authentication.
• DMARC policy for failed messages: DMARC specifications clarify that a p=none policy means no action is taken on messages that fail authentication. While this is less protective, it prevents legitimate emails from being rejected due to misconfigurations during the rollout phase.

Key considerations

• Aligning authentication: For DMARC to pass, documentation indicates that either SPF or DKIM must pass and align with the From domain. Any service that modifies the email must ensure this alignment is maintained. Here's a simple guide to DMARC, SPF, and DKIM.
• Impact of content: While not directly an SPF/DKIM issue, documentation on email security best practices suggests that poorly managed email signatures or excessive content can introduce vulnerabilities or trigger spam filters, affecting overall email trustworthiness.
• Proactive security measures: IT teams are advised to implement strong email security practices, including careful DMARC configuration and monitoring, to prevent issues like phishing and data breaches, which can be exacerbated by unmanaged email components. Read about email security best practices for IT teams.