@jwz
As a slightly cheerier note, it's *slightly* more distributed outside the US, mostly thanks to the lingering effects of ISP email. I'm Membership Secretary for a small sports group (~70 members) and our mailing list is dominated by US providers, but no single provider has more than 20%:
* 18.1% Gmail
* 13.8% Yahoo (.co.uk and .com)
* 13.8% M$ (Mostly hotmail, also outlook. and live.)
* 11.7% BTInternet
* 8.5% AOL (yeah, really)
* 5.3% iCloud/mac.com
* 4.3% Sky
* 2% Protonmail
* 2% Talktalk
* ~21% everything else
A couple of the remainder are personal/work domains on M365, but there's also a gmx.com and some legit indy stuff (email attached to web hosting in cpanel).
I suspect outside the Anglosphere there are a lot of local alts as well - orange in France, GMX in Germany, etc. Heck, the Germans have their whole own version of Linkedin (xing.com), which is big through Germany, Austria & Switzerland.