How do you messure this? Every university in my country has thousands of mail accounts that are constantly used and having worked at an university IT department I have never heard of any numbers collection coming by to ask us how many mails our users sent to each other.

I wouldn't be surprised if that percentage was made up by the big ones to give you the feeling that self hosting mailservers is useless.

You're probably right, I hadn't considered the volume of mail being sent within institutions that might host their own mailservers, so maybe we should say more than like 50-60%.

Is it still a good example of federation though if a vast majority (99%?) of mails between such institutions or outside of orgs large enough to have a campus, tend to go through Microsoft/Google/Apple/Amazon?