In the Microsoft 365 admin center, choose Admin centers > Exchange.
In the EAC, go to Mail flow > Rules and select New + > Create a new rule.
In Name, type a name for the rule, such as Encrypt mail for [email protected].
In Apply this rule if, select a condition, and enter a value if necessary. For example, to encrypt messages going to [email protected]:
- To select an existing name, select it from the list and then click OK.
- To enter a new name, type an email address in the check names box and then select check names > OK.
To add more conditions, choose More options and then choose add condition and select from the list.
For example, to apply the rule only if the recipient is outside your organization, select add condition and then select The recipient is external/internal > Outside the organization > OK.
To enable encryption using the new OME capabilities, from Do the following, select Modify the message security and then choose Apply Office 365 Message Encryption and rights protection. Select an RMS template from the list, choose Save, and then choose OK.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/compliance/define-mail-flow-rules-to-encrypt-email?view=o365-worldwide