Encrypt email messages
When you need to protect the privacy of an email message, encrypt it. Encrypting an email message in Outlook means it's converted from readable plain text into scrambled cipher text. Only the recipient who has the private key that matches the public key used to encrypt the message can decipher the message for reading. Any recipient without the corresponding private key, however, sees indecipherable text.
In an email message, choose Options, select Encrypt and pick the encryption that has the restrictions you want to enforce, such as Encrypt-Only or Do Not Forward.
Encrypt a single message
In message that you are composing, click File > Properties.
Click Security Settings, and then select the Encrypt message contents and attachments check box.
Compose your message, and then click Send.
Encrypt all outgoing messages
When you choose to encrypt all outgoing messages by default, you can write and send messages the same as with any other messages, but all potential recipients must have your digital ID to decode or view your messages.
On the File tab. choose Options >Trust Center > Trust Center Settings.
On the Email Security tab, under Encrypted email, select the Encrypt contents and attachments for outgoing messages check box.
To change additional settings, such as choosing a specific certificate to use, click Settings.