The Exchange Sender Reputation filter must be enabled.

By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Sender Reputation is antispam functionality that blocks messages according to many characteristics of the sender. Sender Reputation relies on persisted data about the sender to determine what action, if any, to take on an inbound message. This setting enables the Sender Reputation function.

Note: If third-party anti-spam product is being used, the anti-spam product must be configured to meet the requirement.

Open the Exchange Management Shell and enter the following command:

Get-SenderReputationConfig | Select Name, Enabled

If the value of "Enabled" is not set to "True", this is a finding.

Open the Exchange Management Shell and enter the following command:

Set-SenderReputationConfig -Enabled $true