If S/MIME is enabled at your site, all encrypted or signed messages are indicated with icons in the header panel of your message.
A PEN icon indicating that the message is signed.
A KEY icon indicating that the message is encrypted.
If you have a valid private certificate, the incoming messages are decrypted and displayed in plain text.
Note - Attachments that are part of the encrypted or signed messages are not displayed inline in the message. They remain as attachments.