The answer is a mail flow rule (transport rule) in Exchange Online. This is the correct feature because it operates on messages in transit, allowing you to apply the action “Prepend the subject line with the string [External]” when the condition “The sender is located outside the organization” is met. On the Microsoft 365 Administrator MS-102 exam, this question tests your understanding of how to tag external emails with a label in the subject line, often appearing as a scenario where you must distinguish between Purview Data Loss Prevention policies, which block or warn but do not modify subject lines, and mail flow rules, which are the only mechanism for direct subject-line manipulation. A common trap is choosing an anti-phishing policy or a sensitivity label, but those do not prepend text to the subject. Remember the memory tip: “Transport rules transport text to the top of the subject,” so when you need to automatically tag external emails with ‘[External]’ in the subject, think of the rule that moves mail through the pipeline.
MS-102 Deploy and manage a Microsoft 365 tenant Practice Question
This MS-102 practice question tests your understanding of deploy and manage a microsoft 365 tenant. 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.
You are configuring Microsoft Purview for your organization. You need to ensure that all external emails are automatically tagged with an 'External' label in the subject line. Which feature should you configure?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Mail flow rule (transport rule) to prepend '[External]' to subject lines.
To automatically prepend a text tag like '[External]' to the subject line of all incoming external emails, you must use a Mail flow rule (also known as a transport rule) in Exchange Online. This rule can be configured with the condition 'The sender is located outside the organization' and the action 'Prepend the subject line with the string [External]'. This is the only mechanism that directly modifies the subject line of emails in transit.
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.
✗
Anti-phishing policy in Defender for Office 365.
Why it's wrong here
Anti-phishing policies handle impersonation and spoofing, not external tagging.
✓
Mail flow rule (transport rule) to prepend '[External]' to subject lines.
Why this is correct
Mail flow rules can modify message properties including subject.
Related concept
Read the scenario before looking for a memorised answer.
✗
Sensitivity label policy to automatically apply an 'External' label.
Why it's wrong here
Sensitivity labels do not modify subject lines.
✗
Data loss prevention (DLP) policy for external emails.
Why it's wrong here
DLP policies detect and protect sensitive data, not modify subject lines.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse the ability to add a visual 'External' tag with sensitivity labels or anti-phishing policies, but only a mail flow rule can directly manipulate the subject line of an email in transit.
Detailed technical explanation
How to think about this question
Mail flow rules in Exchange Online operate on SMTP messages at the transport layer, allowing actions like 'Prepend subject' which directly modifies the RFC 5322 Subject header field. This is distinct from client-side rules or add-ins, as it occurs before the message reaches the user's mailbox. In a hybrid or multi-geo environment, the rule must be scoped correctly to apply to all inbound messages from external senders, and the prepended string is added exactly as configured, including any brackets or spaces.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this MS-102 question in full detail.
Deploy and manage a Microsoft 365 tenant — This question tests Deploy and manage a Microsoft 365 tenant — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Mail flow rule (transport rule) to prepend '[External]' to subject lines. — To automatically prepend a text tag like '[External]' to the subject line of all incoming external emails, you must use a Mail flow rule (also known as a transport rule) in Exchange Online. This rule can be configured with the condition 'The sender is located outside the organization' and the action 'Prepend the subject line with the string [External]'. This is the only mechanism that directly modifies the subject line of emails in transit.
What should I do if I get this MS-102 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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