Embed Encryption to Email and Activity History Emails

Emails and time management tools play an important role in your day-to-day workflow. For another layer of security and compliance controls, you can now encrypt Email and Email to Case messages. You control the life cycle of the keys used to encrypt your emails, helping you meet your compliance and regulatory obligations.

With the Encrypt Your Email to Case Feed and Email Messages, you can encrypt these data elements in the email message.  

• From Name
• From Address
• To Address
• CC Address
• BCC Address
• Subject
• Text Body
• HTML Body
• Headers

You can also encrypt the Relation Address field on the Email Message Relation object.

If you use Email-to-Case, these fields are also encrypted on the customer emails that generate cases.

Ref : https://help.salesforce.com/articleView?id=sf.security_pe_standard_fields.htm&type=5