The numerous other email providers are... numerous. Every discussion like this ignores, to an absurd extent, how hard it is for non-tech people to gather information on these topics and make an informed choice: Information about which email providers are care about which aspects of privacy, which aspects of privacy and information security even exists, which email providers even exist, what they are doind with your data, what parts of what they are doing is a problem...

You can't even ask tech people to make a choice for you because they all say different things.

Other domains like cars, medicine, construction, whatever have established standards because they have recognized that individuals simply _cannot_ make an informed choice, even if they want. I'm eager to say that only information technology likes to call the user "unwilling" and "lazy" instead, but actually individuals from other domains do that too. Luckily, the established standards are mandatory, so their opinion doesn't count.

Rounding error that doesn't matter, because the recipients of any e-mail sent from those providers are likely on mailboxes backed by Google or MSFT anyway.