Set the EMAILPREFERENCESAUTOBCC to have a default value of true
|Knowledge Article Number||000212594|
|Description||Can I set the EMAILPREFERENCESAUTOBCC field on the User record to have a default value of true?|
|Resolution||There is no standard way to automatically set the "Auto BCC" field on the user record to "False" (or off, or unchecked).
However, this is a field that you can edit with the APEX Data Loader (it's referred to as "EMAILPREFERENCESAUTOBCC". You can do an export of all users (making sure to get the User IDs in the export) , set the AutoBCC to "False" and then update via Data Loader.
Keep in mind however, that this is outside of standard functionality and is not supported,endorsed, or recommended by Salesforce.