A good part of that is episodic and accidental. There have been instances of those "spammy" internal mails that just can't give up, such as a mail sent by the HR to inform people that a car was left with lights on in the morning with everyone (and not just the building) in cc instead of bcc or a contact group, with endless streams of "reply all" asking to remove them from the mailing list.

These happen, and in very large companies these issues can degenerate fast unless your email admins are on their toes. Repeat the incident once a year and it's fine. Repeat it once a month and you have a failing email system with very visible streaks of 200+ mails/day.