You can create your own process using the Text Classification functionality applied in the ICM Email Listener workflow. Emails received and processed by the email listener are analyzed and based on the text classification result, the system can then decide whether an existing case needs to be updated or not.
For more details on Text Classification, see this article.
Customize case update based on text classification
You can configure a parameter in the ICM Email Listener workflow to determine whether an existing case needs to be updated when receiving a new email or not.
IsCaseUpdateByEmail does not affect the creation of new cases based on the received emails.