Defining rules for Microsoft Purview Message Encryption
One way to enable Microsoft Purview Message Encryption is for Exchange Online and Exchange Online Protection administrators to define mail flow rules. These rules determine under what conditions email messages should be encrypted. When an encryption action is set for a rule, any messages that match the rule conditions are encrypted before they're sent.
Mail flow rules are flexible, letting you combine conditions so you can meet specific security requirements in a single rule. For example, you can create a rule to encrypt all messages that contain specified keywords and are addressed to external recipients. Microsoft Purview Message Encryption also encrypts replies from recipients of encrypted email.
Reference:
https://learn.microsoft.com/en-us/purview/ome