https://www.bleepingcomputer.com/news/security/office-365-now-opens-attachments-in-a-sandbox-to-prevent-infections/

Microsoft today announced the launch of Application Guard for Office in public preview to protect enterprise users from threats using malicious attachments as an attack vector.

Application Guard for Office (also known as Microsoft Defender Application Guard for Office) is designed to help prevent block files downloaded from untrusted sources from gaining access trusted resources by opening them within an isolated sandbox.