The correct answer is Elevated risk delivery pool.

Exchange Online Protection isolates potentially compromised mailboxes to protect the shared sending reputation and the Elevated risk delivery pool is the tier used for the most suspicious outbound traffic. Messages routed to the Elevated risk delivery pool are subject to stronger throttling and reputation controls so healthy tenant traffic is not harmed while administrators investigate the source of the spam.

No risk delivery pool is incorrect because that pool is for mail that shows no indicators of compromise and would not be used for suspected outbound spam.

Moderate risk delivery pool is incorrect because that tier is for medium risk activity and not for the high risk behavior that warrants isolating traffic to preserve service reputation.

Low risk delivery pool is incorrect because that pool is meant for minor concerns and not for the suspected compromise scenarios that require the elevated risk treatment.

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