A Signed message contains the Certificate of the signer. The Take Certificate button that appears on the Message page when a Signed message is displayed. By clicking that button you include the E-mail address and the name of the signer (as specified in the Certificate, not in the message headers), and the signer certificate into your currently selected Address Book.
When an Address Book is displayed, the [@] marker indicates the entries that have known (stored) certificates. You can send encrypted messages to those addresses.