why would anyone set up AD on an exchange server?

would’nt this be a security nightmare especially since exchange is the garbage filter for the internet and antivirus\antispam is only as good as it’s last update..

I’m getting techs suggesting to put AD on the exchange server and I’m totally against it, especially since I’ve been in the security field for a number of years now.. I would thing exchange is the first think that gets hit when scanners cannot pick up the latest threats.