To Send an encrypted message, make sure that your Private Key is unlocked, and that all message recipients are included into your Address Book, and their Address Book entries contain certificates.
If your Private Key is unlocked, you will see the Send Encrypted checkbox on your Compose page. Select this checkbox to encrypt your message. If you send a message with attachments, the entire content of your message, including all attachments, will be encrypted with the recipients Public Keys (taken from their Certificates), and with your own Public Key. As a result, if a copy of the encrypted message is stored in your Sent Mailbox, you will be able to read (decrypt) it.
If you select both Send Signed and Send Encrypted options, the message will be composed as a Signed message, and then the entire content (including the message headers and your signature) will be encrypted.
Use the Encryption Method WebUser setting to specify the encryption "cipher".