Or, you know, similar reasons as why people use Cloudflare as a reverse proxy.

I would strongly doubt (though not completely rule out) that Microsoft is reading your personal mail hosted on your personal mail server. Like what would that even look like, their IMAP client is forwarding email to headquarters?

BUT, more likely, it's some sort of "check in" or "dial home" feature that isn't working. Or plausibly, maybe they count "local user checked email" and their analytics endpoint is down.

Regardless, it's definitely surprising that a traditional desktop application would be blocking when only polling from local/trusted data sources.

It probably is a mirror for caching purposes for the mobile app or something. It lets them control the protocol between the client and their server as well.

But a check in or analytic counter wouldn’t stop you from receiving email. This is most likely due to Microsoft’s server (not your local computer) being the one fetching the mail.

It is normally probably more reliable; except when their server is down.

They proxy the whole ball of wax. Have done for years.

My understanding is: the IMAP client is hosted. The local app talks to a HTTP service, and doesn't have mail processing capabilities of its own.

[delayed]