I can not figure out where odoo is automatically subscribing a new follower when an email is received in Sales Order (sale.order).

I have outgoing emails setup to not include the recipient as a follower, however, when they reply they are automatically added (subscribed) as a follower of that particular sale order document.

Does anyone know where I can override or stop that action, I suspect it is in mail.thread but I can not figure it out, any help would be greatly apricated.

Thank you!