To automatically encrypt email messages using Microsoft Purview Message Encryption (OME), administrators must configure mail flow rules (also known as transport rules) in the Exchange admin center.
These rules can be configured to check conditions, such as when a recipient is a specific user or when an email contains attachments, and then apply encryption automatically. Sharing policies, Safe Attachments policies, and retention label policies are not used for OME encryption.
Reference: Define mail flow rules to encrypt email messages