Quick Answer
The answer is that the email was forwarded by an intermediary that strips the DKIM signature. This is correct because DKIM works by attaching a cryptographic hash of the original message body and selected headers; when a forwarding server, mailing list, or email relay modifies the content or headers—even legitimately—the hash no longer matches, causing the DKIM verification to fail. On the Fortinet NSE 7 Advanced Security NSE7 exam, this scenario tests your understanding of how email authentication mechanisms interact with forwarding, and it often appears as a trap where candidates assume SPF passing means the email is fully authenticated. The key insight is that SPF checks the sending server’s IP, which can remain authorized after forwarding, but DKIM is broken by any alteration, leading to a DMARC policy that marks the email as spam. Memory tip: “Forwarding fractures the fingerprint”—if the message changes, DKIM’s cryptographic fingerprint breaks, even when the sender is legitimate.
This NSE7 practice question tests your understanding of advanced threat protection. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An administrator configures email authentication (SPF, DKIM, DMARC) on FortiMail. They find that legitimate emails are being marked as spam by FortiMail. The SPF check passes but DKIM fails. What could be the issue?
The SPF record is too strict
Why wrong: SPF passed, so it's not the issue.
The email was forwarded by an intermediary that strips the DKIM signature
Forwarding often breaks DKIM, causing it to fail.
FortiMail has a bug in the DKIM verification module
Why wrong: Unlikely; common issue is forwarding.
The DMARC policy is set to reject
Why wrong: DMARC policy doesn't cause DKIM failure; it only dictates action.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The email was forwarded by an intermediary that strips the DKIM signature
Option B is correct because when an email is forwarded by an intermediary (e.g., a mailing list or forwarding service), the intermediary often modifies the message headers or body, which invalidates the DKIM signature. Since DKIM relies on a cryptographic hash of the original message content and selected headers, any alteration—even by a legitimate forwarder—causes the signature verification to fail. The SPF check passes because the forwarding server may be authorized in the SPF record, but DKIM failure triggers spam classification if the DMARC policy is not aligned.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The SPF record is too strict
Why it's wrong here
SPF passed, so it's not the issue.
The email was forwarded by an intermediary that strips the DKIM signature
Why this is correct
Forwarding often breaks DKIM, causing it to fail.
Related concept
Read the scenario before looking for a memorised answer.
FortiMail has a bug in the DKIM verification module
Why it's wrong here
Unlikely; common issue is forwarding.
The DMARC policy is set to reject
Why it's wrong here
DMARC policy doesn't cause DKIM failure; it only dictates action.
Common exam traps
The trap here is that candidates assume DKIM failure is always due to a misconfiguration on the sending side, rather than recognizing that forwarding or intermediary modification is a common and legitimate cause of DKIM breakage.
Detailed technical explanation
DKIM signatures are created by the original sending domain's MTA using a private key, and the signature covers specific headers (e.g., From, Date, Subject) and the message body. When an intermediary forwards the email, it may add headers like 'Received' or 'List-Id', or modify the body (e.g., adding a footer), which breaks the hash. In real-world scenarios, ARC (Authenticated Received Chain) is used to preserve authentication results across forwarders, but if ARC is not implemented, DKIM fails even though the original sender is legitimate.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Advanced Threat Protection — This question tests Advanced Threat Protection — Read the scenario before looking for a memorised answer..
The correct answer is: The email was forwarded by an intermediary that strips the DKIM signature — Option B is correct because when an email is forwarded by an intermediary (e.g., a mailing list or forwarding service), the intermediary often modifies the message headers or body, which invalidates the DKIM signature. Since DKIM relies on a cryptographic hash of the original message content and selected headers, any alteration—even by a legitimate forwarder—causes the signature verification to fail. The SPF check passes because the forwarding server may be authorized in the SPF record, but DKIM failure triggers spam classification if the DMARC policy is not aligned.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This NSE7 practice question is part of Courseiva's free Fortinet certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the NSE7 exam.
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