Exchange Online transport (mail flow) rules are a powerful way to ensure that email from Office 365 tenants to specific recipients are encrypted in a consistent manner. Using rules relieves the need for users to become involved and makes sure that email is protected in a way that recipients can read messages. It's a good way to use the protection features built into Office 365.
https://www.petri.com/exchange-transport-rules-good-way-encrypt-email
