To Send a signed message, make sure that your Private Key is unlocked. If it is unlocked, you will see the Send Signed checkbox on your Compose page. Select this checkbox to sign your message. If you send a message with attachments, the entire content of your message, including all attachments, will be signed with your Private Key and your Certificate will added to the message signature.
Recipients of your Signed message will be able to verify that the content has not been altered, and they will be able to store your Certificate and later send you encrypted messages.