You want to lock specific parts of an email, such as the From or Reply-To, to prevent certain users from editing them.
Through roles and various permissions you can prevent editing of the email as a whole. You do not have the ability to allow only partial edit of an email header such as the From/To. The same applies to Sales Insight emails. Edit permissions are granted for the email as a whole
Note: You can prevent people from editing pieces of an email by making those sections non-editable on the template level, but again, this would apply to all users.