--- title: "Unintentional DKIM failures: common message modifications that trigger false positives | DuoCircle" description: "Unintentional DKIM failures: common message modifications that trigger false positives." image: "https://www.duocircle.com/images/og-default.png" canonical: "https://www.duocircle.com/blog/email-security/unintentional-dkim-failures-common-message-modifications-that-trigger-false-positives/" --- Quick Answer DKIM verifies a cryptographic hash of headers and body, so any modification in transit breaks the signature even when no attacker is involved. Common causes of false-positive DKIM failures: line wrapping changes when a relay enforces RFC 5322 or 2045 line-length limits, footer or disclaimer injection by mail servers or compliance gateways, character encoding conversions between 7-bit, 8-bit, and quoted-printable, whitespace normalization, and content modifications by mailing list managers. Mitigations: use the relaxed canonicalization mode (c=relaxed/relaxed) which tolerates whitespace and case changes, keep mailing list software at versions that support ARC to preserve original authentication, avoid post-signing footer injection on the sending side, and watch DMARC forensic reports for systematic dkim=fail patterns from specific intermediaries. Unintentional DKIM failures: common message modifications that trigger false positives Your browser does not support the audio element. [ Download episode](https://media.mailhop.org/duocircle/images/2025/02/Unintentional-DKIM-failures-common-message-modifications-that-trigger-false-positives.mp3) Share [ ](https://www.linkedin.com/sharing/share-offsite/?url=undefined%2Fblog%2Femail-security%2Funintentional-dkim-failures-common-message-modifications-that-trigger-false-positives%2F "Share on LinkedIn") [ ](https://twitter.com/intent/tweet?text=Unintentional%20DKIM%20failures%3A%20common%20message%20modifications%20that%20trigger%20false%20positives&url=undefined%2Fblog%2Femail-security%2Funintentional-dkim-failures-common-message-modifications-that-trigger-false-positives%2F "Share on X/Twitter") [ ](https://www.facebook.com/sharer/sharer.php?u=undefined%2Fblog%2Femail-security%2Funintentional-dkim-failures-common-message-modifications-that-trigger-false-positives%2F "Share on Facebook") [ ](https://reddit.com/submit?url=undefined%2Fblog%2Femail-security%2Funintentional-dkim-failures-common-message-modifications-that-trigger-false-positives%2F&title=Unintentional%20DKIM%20failures%3A%20common%20message%20modifications%20that%20trigger%20false%20positives "Share on Reddit") [ ](mailto:?subject=Unintentional%20DKIM%20failures%3A%20common%20message%20modifications%20that%20trigger%20false%20positives&body=Check out this article: undefined%2Fblog%2Femail-security%2Funintentional-dkim-failures-common-message-modifications-that-trigger-false-positives%2F "Share via Email") ![Unintentional DKIM failures](https://media.mailhop.org/duocircle/images/2025/02/spf-record-generator-3275.jpg) DKIM is highly sensitive to alterations. This sensitivity is what makes **DKIM a robust protocol** against [phishing attacks](https://www.infosecurity-magazine.com/news/mobile-phishing-attacks-surge-16/) attempted by changing the email content while it’s in transit. However, sometimes inadvertent modifications happen in transit, which triggers emails to fail DKIM authentication even if a malicious entity hasn’t altered them. This blog lists the common unintentional modifications that lead to false positives. ## Line wrapping/ line break changes _Line wrapping or line break changes happen when email servers or gateways make some changes in the email’s body by inserting, removing, or adjusting line breaks_. This modification is usually made when an **email relay** has to abide by the ‘per line character limit’ imposed by RFC 5322 or RFC 2045\. These minor adjustments mean nothing for the sender and the recipient because the content remains the same; however, [DKIM](/resources/what-is-dkim) relies on an exact cryptographic hash, and that’s why verification checks fail. [![Email Security](https://media.mailhop.org/duocircle/images/2025/02/spf-record-tester-3698.jpg)](https://media.mailhop.org/duocircle/images/2025/02/spf-record-tester-3698.jpg) ## Whitespace modifications White space modifications refer to the unintended alteration of spaces, tabs, or line breaks in an email’s body while it is in transit. [Mail Transfer Agents](https://debounce.io/glossary/mail-transfer-agent-mta/) (MTAs), [email security](/content/email-security-services) gateways, or [anti-spam filters](/spam-filtering/anti-spam-filters-and-how-they-work/) try making these changes to reformat messages for **consistency or better readability**. Sometimes, systems trim extra spaces, replace tabs with spaces, or normalize multiple spaces into a single space so that the content complies with the format laid down by RFC 5322\. ## Header rewriting or reordering This alteration refers to the inadvertent insertion of headers, modification of existing headers, or reordering of headers once they are signed. This invalidates the [DKIM signature](https://docs.mapp.com/v1/docs/dkim-signature). Common examples are- **Adding ‘Received’ or ‘X-’ headers** during [email routing](https://www.cloudflare.com/learning/email-security/what-is-email-routing/). ## Attachment modifications It’s not unusual for email attachments to undergo modifications during transmission. Security policies, virus scanning, and [mail server](https://www.activecampaign.com/glossary/mail-server) optimizations often make these alterations while inspecting and processing attachments to determine whether they contain malware. These systems also ensure email attachments comply with corporate policies and **MIME standards**. They may replace [infected files with warning messages](https://www.bleepingcomputer.com/news/security/trickbot-malware-mistakenly-warns-victims-that-they-are-infected/), [compress attachments](https://www.computerweekly.com/news/366605874/Phishing-links-becoming-bigger-threat-than-email-attachments), or convert file formats (e.g., .docx to .pdf). [![Email Systems Handle Infected Files](https://media.mailhop.org/duocircle/images/2025/02/spf-record-tester-0986.jpg)](https://media.mailhop.org/duocircle/images/2025/02/spf-record-tester-0986.jpg) While these **changes enhance security**, they can invalidate DKIM signatures if any modified portion was included in the **cryptographic hash** when the email was originally signed. ## HTML to plain text conversions _While most email systems support both HTML and plain text formats to ensure compatibility with different email clients and devices, MTAs and security gateways convert HTML to plain text_. This conversion reduces the [risk of phishing](https://www.securityweek.com/understand-more-about-phishing-techniques-reduce-your-digital-risk/), **enhances deliverability**, and improves compatibility with legacy email clients. This change may sound harmless to you, but for DKIM, it means that the **email structure** is stripped out because of missing [HTML tags](https://www.geeksforgeeks.org/html-tags-a-to-z-list/), inline CSS, embedded images, hyperlinks, and formatting attributes. ## How to reduce false positives? Here’s a quick run-down on what you can do to avoid triggering **DKIM for genuine emails** sent from your domain- - Use the ‘relaxed’ canonicalization (e.g., c=relaxed/relaxed) for both headers and body to allow minor modifications (e.g., extra spaces). - Avoid overly strict DKIM policies that require exact **byte-for-byte matching**. - Minimize post-send processing (e.g., avoid adding disclaimers after DKIM signing). If you need professional help, feel free to get in touch with us or explore [DuoCircle](/) for expert **email security solutions**. ## Topics DKIMemail securitySecurity ![Brad Slavin](https://media.mailhop.org/dmarcreport/images/team/brad-slavin.jpg) Brad Slavin General Manager General Manager at DuoCircle. Product strategy and commercial lead across the email security portfolio. ## Secure your email infrastructure Protect, authenticate, and deliver. Contact our team to find the right solution. [Contact Sales](/contact/) [Explore Products](/products/) Share this article [ ](https://www.linkedin.com/sharing/share-offsite/?url=https%3A%2F%2Fwww.duocircle.com%2Fblog%2Femail-security%2Funintentional-dkim-failures-common-message-modifications-that-trigger-false-positives%2F) [ ](https://twitter.com/intent/tweet?text=Unintentional%20DKIM%20failures%3A%20common%20message%20modifications%20that%20trigger%20false%20positives&url=https%3A%2F%2Fwww.duocircle.com%2Fblog%2Femail-security%2Funintentional-dkim-failures-common-message-modifications-that-trigger-false-positives%2F) [ ](https://www.facebook.com/sharer/sharer.php?u=https%3A%2F%2Fwww.duocircle.com%2Fblog%2Femail-security%2Funintentional-dkim-failures-common-message-modifications-that-trigger-false-positives%2F) Copy Related Articles - [ ![DMARC, SPF, and DKIM](https://media.mailhop.org/duocircle/images/2026/04/spf-record-4526.jpg) DMARC, SPF, and DKIM in 2026: Why Email Authentication Is Now a Regulatory Requirement, Not Just a Best Practice Email Security ](/blog/dmarc-spf-dkim-2026-email-authentication-regulatory-requirement-best-practice/) - [ ![Email Monitoring Tools](https://media.mailhop.org/duocircle/images/2026/05/spf-validator-6720.jpg) Email Monitoring Tools: A Complete Guide to Protecting Your Email Ecosystem Email Security ](/blog/email-monitoring-tools-guide-protecting-your-email-ecosystem-security/) - [ ![Hosted SMTP Servers for High Deliverability](https://media.mailhop.org/duocircle/images/2026/04/buy-smtp-1643.jpg) 13 Best Hosted SMTP Servers for High Deliverability in 2026 Email Security ](/blog/email-security/12-best-hosted-smtp-servers-for-high-deliverability-in-2026/) - [ ![email security techniques](https://media.mailhop.org/duocircle/images/2024/12/spf-record-generator.jpg) 5 efficient email security techniques for advanced persistent threats Email Security ](/blog/email-security/5-efficient-email-security-techniques-for-advanced-persistent-threats/) ## Related Articles [ Email Security 12m DMARC, SPF, and DKIM in 2026: Why Email Authentication Is Now a Regulatory Requirement, Not Just a Best Practice Apr 29, 2026 ](/blog/dmarc-spf-dkim-2026-email-authentication-regulatory-requirement-best-practice/)[ Email Security 5m Email Monitoring Tools: A Complete Guide to Protecting Your Email Ecosystem May 7, 2026 ](/blog/email-monitoring-tools-guide-protecting-your-email-ecosystem-security/)[ Email Security 15m 13 Best Hosted SMTP Servers for High Deliverability in 2026 Apr 8, 2026 ](/blog/email-security/12-best-hosted-smtp-servers-for-high-deliverability-in-2026/)[ Email Security 6m 5 efficient email security techniques for advanced persistent threats Dec 3, 2024 ](/blog/email-security/5-efficient-email-security-techniques-for-advanced-persistent-threats/) ```json {"@context":"https://schema.org","@type":"Organization","name":"DuoCircle LLC","url":"https://www.duocircle.com","logo":{"@type":"ImageObject","url":"https://www.duocircle.com/images/duocircle-logo.png"},"description":"DuoCircle is a portfolio of specialized email products covering protection, authentication, delivery, and routing. 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