This isn't to excuse his conduct in any way shape or form, but I suspect that every high ranking official in the public or private sector keeps a private e-mail account for conducting business off-the-books.
Or just having a personal email address for non-work reasons. They're still human, even if they're high-level elected officials or CEOs, and they won't have that job/office forever.
> "...where they read several dozen emails, some of them that Brennan had forwarded from his government work address and that contained attachments..."
Sorry, but this doesn't sound like "personal email address for non-work reasons"...