I’m a professor. Since this date last year, my email account shows a total of just under 10,000 messages received or sent. This includes spam. I need another 64 years to catch up with this guy!
Recently retired as an IT guy from a university. You guys can generate a lot of email and have reason to hold on to it for historical purposes. It wasn't really all that long ago that email servers and client software required archiving because of limitations of both server and client side software. If the 650K email number is accurate this would likely involve a whole lot of archive files.