Right. Exchange, is a messaging system, and used Microsoft MAPI (2003, 2007). Now it uses RPC over HTTPS (Exchange 2010, 2013, 2016).
And IMAP, unlike POP3, allows the viewing of message headers on a messaging system without having to download the entire message. That way an end user could delete spam and unsolicited mail at the server without consuming a lot of bandwidth, whereas just to see your messages in POP3, everything gets transferred to the end user’s system. So it makes no sense that IMAP alone would download all those messages. I’ll bet Weiner used POP3. However, Microsoft stopped using the POP3 connector in Exchange 2007. You could buy an add in for it... Like Popcon or Pytheas.
Hillary’s IT people were pretty dang stupid the way the set up their server.