Digitally signed messages come with a digital certificate that contains the sender's public key information and can be used to send encrypted messages to the sender using the public key.
Select a digitally signed message, click the From Message menu item in the Mail menu, click the Add to Address Book button in the Pop-up Card Properties dialog box, and add the sender to the specified folder in the Address Book. The card you add will contain a digital certificate.
If the sender's card has been saved in the Address book, just want to add a digital certificate to the original card, click Properties on the Mail menu, and then on the Security tab, click the Add to Address Book button, and the Check Certificate dialog box will pop up to list all the cards that match that certificate's e-mail address. Select the card you want, and click OK to add the digital certificate to the selected card.
If you select the Add sender's certificate to my address Book in the Advanced Settings pair or box in Tools | system settings | Security, select the check box. When you receive a signed message with a digital certificate, Foxmail automatically adds the sender's certificate information to the Address Book.