"I can recall no instance in my time at the National Archives when a high-ranking official at an executive branch agency solely used a personal email account for the transaction of government business," said Mr. Baron, who worked at the agency from 2000 to 2013.
Someone else pointed out that Cheney used mostly personal email for government business, but not solely, so I guess that gets a pass.
Also I saw no rule that requires all official emails use government servers. That's what she is accused of actually breaching. At least according to the summary and posters here. There's very little fact into what she did wrong, and more a focus on blaming her for something, anything, so long as it's plausible.