There is a universal setting inside Salesforce that allows all users to see all emails associated with the parent (typically accounts or contacts). To retain the privacy of your emails, you may need to modify your permissions (if your Salesforce user access was created prior to Mar 22, 2021). To do so, please follow the instructions below.
Log in to Salesforce and click the icon (your avatar) in the upper right corner, then click settings. This will open a new page of settings for you. On the left side in the Quick Find box, type in "Sharing" or navigate to Email -> Einstein Activity Capture -> Sharing Settings. Set this setting to "Don't Share". Once done, your activities with a person or account will now be more private. You will be able to see them but the end user will only see minor details (Event or Email, date, time, with whom) on the activity record.