When you need to block external access to your most sensitive documents, Office 365 Data Loss Prevention policies and sensitivity labels combine to find and protect the documents. A really simple policy is enough to detect and block external access, and is covered by Office 365 E3 licenses. If you have E5 licenses, you can consider auto-label policies to find and protect sensitive documents at scale.
Office 365 offers different ways to apply encryption to important messages. When those messages hold sensitive data known to Office 365, like credit cards or passport numbers, we can define a transport rule or DLP policy to protect outbound email automatically. And while you can define rules and policies through the GUI, PowerShell is available too.
Microsoft thoughtfully provided Office 365 tenants with a Data Loss Prevention template for GDPR personal data. The only problem is that the rules in the template were too sensitive and started to block email.