Petraeus was using gmail, not a government server.
http://www.networkworld.com/article/2223513/data-center/general-david-petraeus-used-clever-gmail-trick-during-affair.html
Petraeus and his mistress were both using the same gmail account. Since Hillary and her adviser used the same email server, they could have used a dummy account in common.
Even that would leave tracks on the email server when they deleted the email and depending on how often backups are done, the email in the draft folder may be saved on a backup server. If there wasn’t a backup server, Hillary may be safe, but if the email address was tracked to a government site, there is a good chance there are backups of everything.
When you delete something on a computer, it doesn’t delete everything until it is written over a bunch of times. She could use some fancy software that would help, but delete by itself doesn’t delete all tracks.
I can purchase space on a server and set up a company website, email addresses, etc. It will be in my company’s name, but the email server will actually be maintained off site. I don’t know if this is what Hillary did or if she actually had an email server at her home (or government site). This is what the Congress Critters need to find out. Usually, there would be an email administrator to fix problems, put in patches, do backups, etc. Large companies maintain their own websites and email servers; smaller companies buy space. I don’t know what Hillary did.
I’m wondering if there is 2 or possibly more servers.
The one in her house, which the SS would clearly know about, and others somewhere else.
Possibly multiple locations.
If it was set up to where no emails were actually sent, just people you communicate with having access, then what would stop them from using multiple external hard drives, one for each person you communicate with?