Don't count the servers. Count the people. Only people who work in tertiary industries tend to have corporate email accounts, and they rarely contain personal data, only data owned by their employers. Many businesses have switched to FAANG services, though, because having an IT department is costly for businesses, and has largely gone out of fashion.
I'm pretty sure the vast majority of email servers are corporate servers, not in FANG datacenters.
3-5 years ago I probably would have agreed with you. Today I'm not so sure. And even among the companies running their 'own' email servers, the physical hardware is very likely in a Microsoft/Amazon/Google datacenter.
"I'm pretty sure the vast majority of email servers are corporate servers"
Many are moving to Office365, and to a lesser degree, Gmail.
Microsoft is making it financially difficult to justify running your own Exchange on-prem.
I'm pretty sure the vast majority of email servers are corporate servers, not in FANG datacenters.