According to the international standard, S/MIME, email subject is not encrypted. Those that are encrypted include email content and attachment while email subject is left in plain text. Therefore, please not include any sensitive information in email subject. Some of us may be used to writing core information in email subject, and now we have to give up this habit for information and privacy security. Maybe, you can use 1 ~ 10 in the subject to indicate the degree of importance of a certain email.
Once again, for some of us write long subjects that contain lots of private or confidential information and in the content they write "As the subject writes", please note that email subject is never encrypted under S/MIME standard. We need to keep our email subject out of any confidential or private information. Such habit of writing an email must be abandoned.