After you have added your certificate, you can sign or encrypt a message by clicking Security > S/MIME Sign or S/MIME Encrypt in the message composer. To have every message signed or encrypted:Click Edit > Preferences, then select the account to encrypt the messages in.Click Edit, then click Security.Click Select next to Signing Certificate and specify the path to your signing certificate.orClick Select next to Encryption Certificate and specify the path to your encryption certificate.Select the appropriate options.Click OK.Click Close.
