To receive an encrypted message, you must first obtain your own certificate from a certifying authority and then give it to the sender. The sender, in turn, must provide you with their certificate, which can be attached to the mail message they send.
Likewise, to receive a digitally signed message, you must have the sender's certificate to verify the digital signature.
Any certificate (.pub) file can be sent as an e-mail attachment and then imported into your Address Book.
After you have completed these steps, when you receive a digitally signed and/or encrypted message, you can open it the same way you would open any other message.
The following topics provide more information: