I think this is a quirk of spoken English that's made its way onto the page as part of the "yes, but" construction. The latter part is emphasized to highlight t

I think this is a quirk of spoken English that's made its way onto the page as part of the "yes, but" construction. The latter part is emphasized to highlight the contrast to the first clause. Yes, these emails are coming from the FBI's infrastructure, but they are not legitimate.