The correct answer is the firewall policy that sources the internal network, destinations the external mail server, uses the SMTP service, and has NAT enabled. This configuration is correct because outbound SMTP traffic from private internal IPs must undergo source NAT—typically masquerading to the FortiGate’s external interface IP—so that the external mail server can route reply packets back through the FortiGate, which then translates the destination back to the original internal host. On the Fortinet NSE 4 Network Security Professional exam, this question tests your understanding of how source NAT integrates with policy-based routing for standard outbound services; a common trap is forgetting that NAT must be explicitly enabled on the policy, even when the service is defined. Remember the mnemonic “SEND” for SMTP outbound: Source internal, Enable NAT, Destination external, service SMTP.
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
A FortiGate administrator needs to allow SMTP traffic from the internal network to an external mail server. The internal network uses source NAT to the external interface IP. Which firewall policy configuration is correct?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Policy: source internal, destination external, service SMTP, enable NAT
Option A is correct because SMTP traffic from the internal network to an external mail server requires source NAT (masquerading) to translate private source IPs to the FortiGate's external interface IP. This ensures return traffic is routed back correctly. The default SMTP service uses TCP port 25, and enabling NAT on the policy is the standard configuration for outbound traffic to the internet.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Policy: source internal, destination external, service SMTP, enable NAT
Why this is correct
SMTP uses TCP port 25 and NAT is needed for outbound traffic.
Related concept
Read the scenario before looking for a memorised answer.
✗
Policy: source internal, destination external, service SMTP, disable NAT
Why it's wrong here
Without NAT, internal private IPs will not be routable to the internet.
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse SMTP ports (25 vs 587) or assume SMTP can use UDP, but the exam tests the fundamental requirement that outbound internet traffic must have NAT enabled and that SMTP is TCP-based.
Detailed technical explanation
How to think about this question
Under the hood, FortiGate's NAT implementation uses IP pool or outgoing interface address (central NAT) to perform source address translation. When NAT is enabled on a policy, the FortiGate rewrites the source IP and port in the packet header, maintaining a session table entry for reverse translation. In real-world scenarios, if the external mail server requires SMTP over port 587 (submission), a separate policy with that service should be created, but the default SMTP service is port 25 TCP.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Policy: source internal, destination external, service SMTP, enable NAT — Option A is correct because SMTP traffic from the internal network to an external mail server requires source NAT (masquerading) to translate private source IPs to the FortiGate's external interface IP. This ensures return traffic is routed back correctly. The default SMTP service uses TCP port 25, and enabling NAT on the policy is the standard configuration for outbound traffic to the internet.
What should I do if I get this NSE4 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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