Encrypting an email in Outlook 2013/365 requires a digital ID to convert the plain text to the ciphered version. Microsoft provides links to various authorities from which you can acquire a free digital ID. With this ID, you can encrypt your emails to protect them from prying eyes. Before the recipient can read the encrypted email message, however, he must possess your public encryption key. You can ensure he has this key by exchanging digitally signed emails and adding the recipient to your contacts list.
Acquiring a Digital Signature
Click the "File" tab and then select "Options."
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Click "Trust Center" and then "Trust Center Settings" in the Microsoft Outlook Trust Center section.
Click "Email Security" and then select "Get a Digital ID" in the Digital IDs (Certificates) section. Your browser will automatically open to Microsoft Office Online and present certificate authorities. Select the one you want, such as "VeriSign" or "GlobalSign," and follow the instructions for that particular authority.
Digitally Signing a Single Email
Click "Options" at the top of the email composition window.
Click "Message Options" in the More Options group.
Click "Security Settings" and then check "Encrypt Message Contents and Attachments." When you send the email, the text and any attachments are then encrypted.
Digitally Signing All Emails
Click "File" and then select "Options."
Click "Trust Center" and then "Trust Center Settings."
Click "Email Security" and then check "Encrypt Contents and Attachments for Outgoing Messages" in the Encrypted Email section.
To select between multiple digital IDs, click "Settings" from the Trust Center's Email Security tab and choose the ID you want to use.