Summary
Microsoft Outlook's 550 5.7.515 "access denied" errors are frequently attributed to DKIM authentication failures, primarily because the email message has been altered after it was initially signed. A significant factor in these failures is Microsoft's unique tendency to modify messages, such as adding headers or "fixing" character encoding, before performing DKIM validation. This pre-validation alteration invalidates the cryptographic signature, leading to rejection. Additionally, any intermediate system-like mailing lists, forwarding services, or security scanners-that modifies an email's headers or body post-signature will also cause DKIM validation to fail. Such integrity breaches cause the receiving server to deem the message unauthenticated or suspicious, resulting in the delivery error.
Key findings
- Increased DKIM Failures: There's been an observed increase in Outlook 550 5.7.515 errors specifically tied to DKIM failures, often indicated by 'Dkim= Fail' in bounce messages, suggesting a heightened scrutiny from Microsoft.
- Encoding & Non-ASCII Impact: Badly encoded characters, accents, or emojis in DKIM-signed headers, particularly in languages like French or Spanish, are frequently linked to these DKIM failures. However, the issue is not exclusive to specific character sets and can also affect standard English messages.
- Microsoft's Message Modification: A critical cause of DKIM failures is Microsoft's long-standing practice of modifying messages-such as adding headers, converting tabs to spaces, or attempting to 'fix' character encoding-before evaluating the DKIM signature, which inevitably breaks the signature's integrity.
- Post-Signature Alterations: Any alteration to the email's headers or body after it has been DKIM signed, whether by intermediate servers, mailing lists, forwarding services, or security appliances, will invalidate the DKIM signature, leading to validation failure.
- DKIM-DMARC-Rejection Cascade: DKIM failures often contribute to DMARC failures, and if a domain's DMARC policy dictates rejection on authentication failure, it directly leads to messages being blocked with errors like 550 5.7.515.
Key considerations
- Review Character Encoding: Thoroughly review and ensure correct character encoding, especially for messages containing non-ASCII characters, accents, or emojis, as issues here are a common trigger for Microsoft's modification behavior that breaks DKIM.
- Identify Intermediate Modifiers: Be aware that email forwarding, mailing lists, security gateways, archiving solutions, and other intermediate services can modify messages in transit, which will break DKIM signatures and should be investigated if errors occur.
- Understand Microsoft's Behavior: Recognize that Microsoft's mail servers have unique and sometimes unpredictable behaviors, including modifying messages before DKIM evaluation, randomly failing DNS, and occasionally misreporting SPF failures when DKIM is the true issue.
- Monitor Deliverability & Authentication: Continuously monitor deliverability and authentication reports to identify patterns of DKIM failures and unexpected rejections, especially those indicating the 550 5.7.515 error.
- Align DMARC Policy: Ensure your DMARC policy is robust and aligned with your authentication practices, as DKIM failures contribute directly to DMARC policy enforcement and subsequent message rejection.
What email marketers say
13 marketer opinions
Receiving a Microsoft Outlook 550 5.7.515 'access denied' error, specifically when linked to a DKIM failure, typically indicates that the email message was modified after it was originally signed. A significant contributor to these issues is Microsoft's tendency to alter incoming messages-for instance, by 'fixing' character encoding, reformatting content, or adding internal headers-before it attempts to validate the DKIM signature. Because the DKIM signature relies on the message's original state, any such post-signing alteration, whether by Microsoft's systems or by intermediate services like mailing lists or security scanners, renders the signature invalid. This integrity breach then prevents the message from passing authentication checks, leading to its rejection.
Key opinions
- Outlook's Message Alterations: Microsoft Outlook's internal systems frequently modify emails by adjusting character encoding, converting tabs to spaces, or inserting headers, often *before* performing DKIM validation, which directly causes otherwise valid signatures to fail.
- Encoding-Driven Failures: Messages with non-ASCII characters, accents, or emojis in DKIM-signed headers are particularly prone to these modifications, although the issue is not exclusive to specific character sets and can also affect standard English messages.
- Intermediate System Interference: Any service or server in the email's transit path, including mailing lists, email forwarding, security gateways, or archiving solutions, can inadvertently modify the email's content or headers post-signing, leading to DKIM signature invalidation.
- Confused Error Reporting: Senders sometimes encounter misleading SPF failure reports for 550 5.7.515 errors when the underlying problem is actually a DKIM validation failure, often due to Microsoft's internal processing or message re-routing, complicating troubleshooting.
- Microsoft's Unpredictable Behavior: Microsoft's mail infrastructure exhibits unique and sometimes unpredictable behaviors, including random DNS issues and unexpected authentication re-routing, contributing to the difficulty of resolving persistent DKIM-related rejections.
Key considerations
- Verify Character Encoding: Proactively ensure correct character encoding, especially for messages containing non-ASCII characters, accents, or emojis, as faulty encoding is a frequent trigger for Microsoft's problematic message modifications.
- Account for Microsoft's Logic: Understand that Microsoft's mail servers may 'fix' or alter messages, such as adding headers or converting character encoding, before evaluating DKIM, making it crucial to anticipate how these pre-validation changes might affect your signature.
- Assess Intermediate Path Effects: Identify and evaluate any third-party services in your email delivery path-including relays, security gateways, mailing lists, and forwarding services-that could be modifying messages and inadvertently breaking DKIM signatures.
- Prioritize Message Integrity: Implement rigorous practices to ensure email message integrity from the point of signing until delivery, recognizing that even minor alterations post-signing will invalidate DKIM and can lead to rejection.
- Distinguish Authentication Failures: Be prepared to thoroughly investigate when 550 5.7.515 errors occur, as bounce messages might attribute the issue to SPF, while the actual underlying problem is a DKIM validation failure due to message modification or Microsoft's unique processing.
Marketer view
Email marketer from Email Geeks explains observing a significant increase in Outlook 550 5.7.515 access denied errors, specifically linked to DKIM failures (Dkim= Fail). He notes this often occurs with French or Spanish language messages, accents, or emojis in DKIM Signed Headers, suggesting it's related to badly encoded characters. He confirms fixing encoding issues resolved the problem and highlights Microsoft's behavior of "fixing" or modifying messages (e.g., adding a Date header) before evaluating DKIM, which can then cause validation failures.
26 Dec 2021 - Email Geeks
Marketer view
Email marketer from Email Geeks explains that DKIM validation can be unreliable when messages contain non-ASCII characters in headers, as this can make the message "not email" in a standard sense, and Microsoft has a history of modifying broken headers during delivery, which in turn breaks DKIM. He reiterates that DKIM failures can stem from odd characters or simply "Microsoft being Microsoft." He also clarifies that if an error specifically states SPF failure, it's an SPF issue, potentially due to Microsoft's internal forwarding confusion.
23 Jul 2024 - Email Geeks
What the experts say
1 expert opinions
The Microsoft Outlook 550 5.7.515 'access denied' errors often arise from DKIM authentication failures, which occur when an email message undergoes modification after its DKIM signature has been applied. DKIM's purpose is to verify the integrity of specific email components; therefore, any post-signing alterations-whether by mailing list managers, automatic footers, or even subtle adjustments to message encoding-will corrupt this cryptographic signature. This breakdown in authentication prevents the message from being verified as legitimate, leading receiving mail servers to reject it or categorize it as spam.
Key opinions
- DKIM's Core Function: DKIM signatures are designed to protect email integrity by cryptographically signing specific headers and a portion of the message body, ensuring the email hasn't been tampered with.
- Post-Signing Invalidation: Any alteration to the signed parts of an email after it has left the sender's mail server will invalidate the DKIM signature, as the cryptographic hash no longer matches the modified content.
- Common Modifiers: Intermediary systems, such as mailing list managers, email forwarding services, or automatic footers, are frequent sources of these post-signing modifications, even if they seem minor.
- Impact of Modifications: Modification to message encoding or the addition of content can directly lead to DKIM signature failure, regardless of the message's initial validity.
- Rejection Consequence: An invalidated DKIM signature causes the email to fail authentication checks, which can lead to rejection or classification as spam by receiving mail servers like Microsoft Outlook, manifesting as errors like 550 5.7.515.
Key considerations
- Identify Modification Points: Pinpoint any intermediate services, such as mailing lists, forwarding systems, or security scanners, that might be modifying your emails after they are signed, as these are common causes of DKIM failures.
- Review Auto-Additions: Scrutinize all automatic additions to your emails, like footers or disclaimers, to ensure they do not alter DKIM-signed content or introduce encoding issues that could invalidate the signature.
- Maintain Message Integrity: Prioritize maintaining the exact integrity of your email content and headers from the moment the DKIM signature is applied until the message reaches the recipient, as even minor changes can break the signature.
- Test End-to-End: Conduct thorough testing of your email paths, especially when using third-party services or mailing lists, to detect any unintended modifications that could break DKIM authentication and lead to delivery errors.
Expert view
Expert from Word to the Wise explains that DKIM signatures protect email integrity by signing specific headers and a portion of the message body. Any modification to these signed parts after the email leaves the sender's mail server, such as changes made by mailing list managers or automatic footers, will invalidate the DKIM signature. This invalidation can cause emails to fail authentication, potentially leading to rejection or classification as spam by receiving mail servers like Microsoft Outlook, thus resulting in delivery errors such as 'access denied' (e.g., 550 5.7.515).
23 Feb 2025 - Word to the Wise
What the documentation says
6 technical articles
The Microsoft Outlook 550 5.7.515 "access denied" error frequently signals a failure in DKIM authentication, which occurs when an email message is altered after its cryptographic signature has been applied. DKIM's core function is to guarantee message integrity; therefore, any modifications to the signed headers or body-ranging from changes in character encoding and text reformatting to the insertion of disclaimers or footers-will invalidate the signature. This breach of integrity means the message cannot be verified as legitimate, leading to its rejection by receiving servers and the subsequent 550 5.7.515 error.
Key findings
- DKIM's Integrity Role: DKIM employs cryptographic signatures to ensure the integrity of an email message, verifying that its content and specific headers remain unchanged from the moment of signing until reception.
- Post-Signing Alteration Impact: Any modification to the email's signed components, whether headers or body, after the DKIM signature is applied will invalidate the signature, as the cryptographic hash will no longer match the altered content.
- Encoding & Formatting Effects: Changes to message encoding, reformatting of text, or the insertion of content such as disclaimers can directly break a DKIM signature, leading to authentication failure.
- Error Causation: A failed DKIM validation is a direct trigger for Microsoft Outlook's 550 5.7.515 'access denied' error, particularly when a domain's DMARC policy mandates rejection on authentication failure.
- Intermediate Modifier Risk: Third-party services, including intermediate servers, mailing lists, or security scanners, can inadvertently modify emails in transit, thereby corrupting DKIM signatures and causing validation failures.
Key considerations
- Preserve Message Integrity: Ensure that email content and headers remain entirely unaltered after the DKIM signature is applied, as even minor changes will invalidate the signature and lead to delivery issues.
- Verify Encoding Practices: Thoroughly review and confirm that your email's character encoding is consistent and correctly handled across all systems to prevent unintended modifications that could break DKIM.
- Audit Transit Path: Identify and audit all intermediate systems, such as mailing lists, forwarding services, or security gateways, that might modify your emails in transit, as these are frequent causes of DKIM failure.
- Monitor Authentication Failures: Continuously monitor for DKIM authentication failures and associated rejection errors like 550 5.7.515 to quickly identify and address root causes related to message modification.
- Align DMARC Strategy: Ensure your DMARC policy is robust and appropriately configured to manage the consequences of DKIM validation failures, which can directly lead to message rejection.
Technical article
Documentation from Microsoft Learn explains that the 550 5.7.515 access denied error indicates a problem with the sender's domain authentication, specifically mentioning SPF, DKIM, and DMARC. It implies that a failed DKIM check can trigger this error, often due to the sending domain not being properly authenticated or seen as suspicious, though it doesn't directly detail encoding or modification effects on DKIM for this specific error.
14 Jul 2022 - Microsoft Learn
Technical article
Documentation from Google Postmaster Tools explains that DKIM uses cryptographic signatures to verify that an email message hasn't been tampered with in transit. If any part of the signed message, including headers or body, is modified after the signature is applied, the DKIM validation will fail. This failure can lead receiving servers, like Outlook, to reject the email with errors such as 550 5.7.515, especially if DMARC policy dictates rejection on DKIM failure.
10 Apr 2023 - Google Postmaster Tools Help
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