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.
What happens if the recipient doesn't have the corresponding private key? The recipient will see this message:
"This item cannot be displayed in the Reading Pane. Open the item to read its contents."
And if the recipient tries to open the item, a dialog box opens with this message:
"Sorry, we're having trouble opening this item. This could be temporary, but if you see it again you might want to restart Outlook. Your Digital ID name cannot be found by the underlying security system."
What else do you need to know about encrypted messages?
Sending and viewing encrypted email messages requires both sender and recipient to share their digital ID, or public key certificate. This means that you and the recipient each must send the other a digitally signed message, which enables you to add the other person's certificate to your Contacts. You can’t encrypt email messages without a digital ID.
If you send an encrypted message to a recipient whose email setup doesn’t support encryption, you're offered the option of sending the message in an unencrypted format.
Any attachments sent with encrypted messages also are encrypted.
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.
