Can email signatures, especially via Exclaimer, cause SPF or DKIM failures and impact email delivery?
Matthew Whittaker
Co-founder & CTO, Suped
Published 26 Jun 2025
Updated 17 Aug 2025
9 min read
The question of whether email signatures, particularly those managed by server-side solutions like Exclaimer, can cause SPF or DKIM failures and consequently impact email delivery is a common concern. It's a nuanced issue, as these services are designed to integrate seamlessly with existing email infrastructure, yet their modifications to email content and routing can sometimes lead to unexpected authentication challenges.
Server-side signature solutions operate by intercepting emails, applying signatures, and then relaying them. This process involves altering the email, which can inadvertently affect its authentication validity. Understanding the fundamentals of SPF and DKIM is crucial to pinpointing where these failures might occur.
My goal here is to clarify how these services interact with email authentication protocols and to provide practical steps for diagnosing and resolving any related deliverability issues you might encounter.
Before diving into how signatures affect authentication, it's important to grasp the roles of SPF and DKIM. Sender Policy Framework (SPF) is a DNS record that specifies which mail servers are authorized to send email on behalf of your domain. When a receiving server gets an email, it checks the SPF record to see if the sending IP address is listed as legitimate. If it's not, the email may fail SPF authentication.
DomainKeys Identified Mail (DKIM) adds a digital signature to your outgoing emails, which is then verified by the recipient's mail server using a public key published in your DNS. This signature ensures that the email content hasn't been tampered with in transit and that the sender is indeed who they claim to be. A DKIM failure often indicates a modification to the email body or headers after the signature was applied.
These protocols work together with DMARC to provide a comprehensive email authentication framework. If either SPF or DKIM fails, DMARC policies dictate how receiving servers should handle the email, ranging from no action to quarantine or rejection.
Server-side signature processing and authentication
Server-side email signature solutions, such as Exclaimer Cloud, function by routing all outgoing emails through their own servers before they reach the final recipient. During this process, the service adds the designated email signature, ensuring consistency across an organization. This interception and modification are precisely where the interaction with email authentication protocols occurs.
When an email leaves your Microsoft 365 or Google Workspace environment, it might initially have a DKIM signature applied by your mail provider. However, when a service like Exclaimer intercepts and alters the email to add a signature, this modification can invalidate the original DKIM signature. For this reason, Exclaimer is designed to either strip out and reapply DKIM, or ensure that Microsoft 365 reapplies a valid DKIM signature after the message has been processed by the signature service and before it's sent to the recipient server. Proper configuration of your SPF record is also essential, as the email is now being sent from an intermediate server. Your SPF record must include the IP ranges or domains of the signature service. You can find specific guidance on preventing emails from being marked as spam on the Exclaimer support pages.
The potential for SPF or DKIM failures arises if these re-authentication steps aren't properly configured or if there are conflicts with existing DNS records. Issues such as broken SPF records with too many DNS lookups can lead to authentication problems and impact deliverability.
Local processing
When users add signatures directly within their email client, such as Outlook, the signature is part of the email content before it leaves the sender's mail server. This means the email is signed for DKIM purposes with the signature already included, reducing the risk of authentication breaks due to content modification in transit.
Cloud processing
Server-side solutions like Exclaimer intercept emails after they leave your mail server but before they reach the recipient. They add the signature and then re-send the email. This post-signing modification can break DKIM unless the service, or your mail provider, correctly re-signs the email, or unless the SPF record is updated to include their sending servers.
Potential issues with email signatures
Even with correct SPF and DKIM configuration, server-side signatures can sometimes introduce subtle issues that affect deliverability. One such issue arises when the signature service embeds elements like un-whitelabeled tracking links or excessively large base64 encoded images. While these don't directly cause SPF or DKIM failures, they can trigger spam filters and cause deliverability issues, potentially redirecting emails to the junk folder or blocking them outright.
The sheer volume of changes a signature service can make to an email, such as encoding the entire plain text and HTML as base64 or adding significant data for tracking, might make a technically valid email appear suspicious. Enterprise mail filters, in particular, are configured to identify and block patterns associated with spam or phishing, even if authentication checks pass.
How Exclaimer modifies emails
Exclaimer's process can involve embedding substantial base64 encoded images (e.g., 70KB) and adding links to its own domain within the email content. While functional, if these external domains are ever associated with unwanted email, even through no fault of your own, it could negatively impact your emails' standing with recipient servers. This can result in emails being marked as spam or even triggering a blocklist (or blacklist) for your domain.
Another area of concern is email forwarding. If your emails are being forwarded after passing through Exclaimer, this additional routing can further break SPF or DKIM due to further header or body modifications. This scenario makes DMARC enforcement crucial, but also more complex to manage, as the original authentication may no longer align with the sender after forwarding. For more information, read about how email forwarding affects authentication validation.
Troubleshooting and remediation steps
The first step in diagnosing any email deliverability problem is always to gather detailed data. Relying on anecdotal reports of delivery problems without specific rejection messages or DMARC reports makes effective troubleshooting nearly impossible. Request copies of bounce messages or, if emails are landing in spam folders, ask a friendly recipient to inquire with their email administrator about the specific reasons. You can understand and troubleshoot DMARC reports to get more insight.
Once you have data, verify your SPF record to ensure it includes Exclaimer's sending IP addresses or domains. Similarly, confirm that Microsoft 365 DKIM signing is correctly configured to apply signatures after Exclaimer's processing. If DKIM is failing, it often points to a mismatch between the content that was signed and the content that was received. You can also review our guide on fixing Microsoft Office 365 DKIM failures.
If SPF and DKIM authentication checks are passing, then the issue is likely not related to the core authentication protocols themselves. Instead, it could stem from the content of the email, including the signature. Factors like the size of the email (due to embedded images), the ratio of images to text, or suspicious links (even if legitimate for the signature service) can lead to emails landing in spam even with perfect authentication. In such cases, a broader review of your email content and overall sending practices may be necessary.
Ultimately, if SPF and DKIM are passing, but you're still experiencing deliverability issues, the problem likely lies beyond basic authentication. This could include issues with your sender reputation, content, or recipient-specific filtering rules. It's crucial to obtain granular data and involve technical teams to investigate the full mail flow and any specific rejection messages.
Maintaining email deliverability with signatures
Email signatures, particularly those implemented via server-side solutions like Exclaimer, do not inherently cause SPF or DKIM failures if they are correctly configured. These services are designed to manage authentication during their processing. However, any modification to an email can complicate its authentication path, requiring careful attention to DNS records and mail flow.
The key to maintaining strong email deliverability is to ensure that your SPF, DKIM, and DMARC records are correctly set up to account for all sending services, including signature management platforms. Regular monitoring of your email authentication status and deliverability reports will help you quickly identify and address any issues that arise.
When facing deliverability challenges, avoid making assumptions about the root cause. Instead, focus on obtaining concrete data, such as bounce messages or DMARC reports, and work with your IT team or email experts to thoroughly investigate the mail flow. This methodical approach is key to resolving complex deliverability problems and ensuring your emails consistently reach the inbox.
Views from the trenches
Best practices
Ensure your SPF record explicitly includes the sending IP ranges or domains of your server-side signature provider.
Verify that your DKIM signatures are correctly applied by your mail service after the signature solution has processed the email.
Regularly monitor your DMARC reports to identify any authentication failures and troubleshoot them promptly.
Review email content, including signatures, for elements that might trigger spam filters, such as excessive images or un-whitelisted links.
Common pitfalls
Failing to update your SPF record to include your signature provider, leading to SPF failures.
Assuming DKIM will automatically pass after a server-side signature modification without verifying re-signing.
Ignoring DMARC reports, missing critical insights into authentication issues and policy enforcement.
Overlooking the impact of email content size and complexity, especially with embedded images, on spam filters.
Expert tips
Confirm that your DKIM signature is applied with your root domain by Microsoft 365, not the initial *.onmicrosoft.com domain.
If your emails contain links to the signature provider's domain that are not whitelabeled, these might contribute to deliverability issues if that domain has a poor reputation.
Be aware that large base64 encoded images within signatures can cause emails to appear suspicious to enterprise filters, potentially leading to filtering.
If receiving issues coincide with sending problems, the root cause might be broader than just your outbound signature configuration.
Expert view
Expert from Email Geeks says that email forwarding can indeed lead to broken signatures and subsequent authentication failures.
July 10, 2024 - Email Geeks
Expert view
Expert from Email Geeks says that Exclaimer modifies email content, which necessitates that the DKIM signature be reapplied after its processing to maintain valid authentication.
Microsoft 365 or 
Google Workspace environment, it might initially have a DKIM signature applied by your mail provider. However, when a service like Exclaimer intercepts and alters the email to add a signature, this modification can invalidate the original DKIM signature. For this reason, Exclaimer is designed to either strip out and reapply DKIM, or ensure that Microsoft 365 reapplies a valid DKIM signature after the message has been processed by the signature service and before it's sent to the recipient server. Proper configuration of your SPF record is also essential, as the email is now being sent from an intermediate server. Your SPF record must include the IP ranges or domains of the signature service. You can find specific guidance on preventing emails from being marked as spam on the Exclaimer support pages.
Outlook, the signature is part of the email content before it leaves the sender's mail server. This means the email is signed for DKIM purposes with the signature already included, reducing the risk of authentication breaks due to content modification in transit.
Exclaimer intercept emails after they leave your mail server but before they reach the recipient. They add the signature and then re-send the email. This post-signing modification can break DKIM unless the service, or your mail provider, correctly re-signs the email, or unless the SPF record is updated to include their sending servers.
