I think what the OP wants (and what I want) is for emails to be logged in the CRM under any contacts that exist, and not create new contacts if they don’t exist.
For example: I might be happily emailing back and forth with a contact. Then suddenly they or I decide to copy in 100 other people. I still want the email logged in CRM under my contact but I don’t want the 100 new contacts created in CRM.
You may say that I can tick or untick the “Log in CRM” box but, as I understand it, this doesn’t help if contacts I don’t want are also on the email chain or where I don’t know if the contact is already on CRM or not (generally if they are I do want it logged, if they are not I don’t).