You can read digitally signed and/or encrypted messages like any other message. Outlook Express displays a help screen the first time you open or preview a digitally signed message and/or an encrypted message.
If you receive a secure message that has a problem, such as the message was tampered with or the digital ID of the sender is expired, you will see a security warning that details the problems before you are allowed to view the contents of the message. Based on the information in the warning, you can decide whether to view the message.
After you send a digitally signed message to a contact, you can read an encrypted message from them the same way you read a regular message.