In accordance with the S/MIME standard, the email subject is not encrypted. Only the email body (including attachments) is encrypted. Therefore, it is recommended not to write sensitive information in the subject, the original habit of directly writing the core content of the email in the subject needs to be changed. It is recommended that it’s enough to only simply mention the email content in the subject, or just write 1-10 to indicate the priority of the email.
When you receive an encrypted email, you can sign in your mailbox on a browser, and if the email is really encrypted, then you cannot browse it, and the message becomes an attachment or simply can’t be display. If you view your encrypted messages with other email client software that does not support S/MIME encryption, you will be prompted “cannot open the message or cannot decrypt". If you use Outlook and have imported your encrypting certificate, Outlook can decrypt your encrypted messages correctly.