To exchange encrypted messages, you must first obtain your own digital ID from a certifying authority (see the Microsoft Internet Explorer Digital ID site for a link to a certifying authority's Web site). and then give the "public key" portion of the digital ID to the people you want to correspond with.
By default, Outlook Express sends your public key whenever you send a digitally signed message. To check this, follow these steps:
After you send a digitally signed message with your digital ID to a contact, you can read an encrypted message from them the same way you read a regular message.