From the Project/Properties menu, click on the S/MIME tab:
Digitally sign messages: You can send signed messages
in clear mode or in opaque mode. In clear mode, the digital
signature is added at the end of the message; in opaque
mode, the message is coded. If you send signed messages in
opaque mode, users of email programs which do not handle
the S/MIME protocol will not be able to read your signed
messages.
Encrypt content and attachments: If you check this
option, messages will be encrypted by using the specified
algorithm. In order to encrypt a message, you must have the
public part of the receiver certificate (public key).
