If she was using POP3, then she wouldn’t be able to check her email anywhere other than where she happened to be logged into. I imagine someone as high up in the IRS as Lerner travels quite a bit. Even if she was using a PST file, she’d have to have it in a central server location so she could access it from other IRS locations.
Not necessarily. She might have used her Blackberry to read email as well as her laptop or desktop computer. And if she used a portable computer, and took it with her, she'd have access to her email wherever she happened to be.
As another poster pointed out, its time for someone from the IRS IT department to testify regarding the architecture of their email system, their backup procedures, and how they met their legal requirement to maintain archival copies of emails.