What action should be taken in a Microsoft Defender for Office 365 Safe Attachments policy to ensure no malware gets delivered to user mailboxes?

Selecting to block emails and attachments with detected malware within the Microsoft Defender for Office 365 Safe Attachments policy is the most effective action to prevent malware from reaching user mailboxes. This policy setting ensures that any email identified as containing malware will not only be prevented from being delivered but will also stop any future emails from the same source with similar threats. By employing this preventive measure, organizations can safeguard their digital environment, maintain data integrity, and reduce the risk of malware infections that could compromise sensitive information or disrupt operational processes.

This action is critical as it actively protects users from threats before any potential damage occurs. In contrast, the other options, such as monitoring or dynamic delivery, may still allow the delivery of messages that could contain harmful content until a full inspection is completed, which can leave windows of vulnerability. Therefore, the choice to block ensures a proactive and robust defense against malware in an organizational context.