Yes, then they use Outlook for example. Have you checked the ToS of the new Outlook version for commoners? They flat out state that they can use all of your emails for whatever they want.

Also, companies host for example an Exchange server on prem; and guess, what it connects to? Why you can usually access account at outlook.com?

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consonaut a day ago | parent [-]

Your on premise exchange server has zero connections to outlook.com. OWA (Outlook Web Access) looks similar to outlook.com but has otherwise nothing to do with it.

	▲	ruszki 18 hours ago \| parent [-]
		Then let’s call outlook.office.com, and that’s an OWA, and you’re redirected there if you login on outlook.com. And that’s the exception when on prem Exchange servers really work as more than mere proxies nowadays. I’m quite sure that there are still real fully on prem solutions, but it’s laughable these opinions, that most company really care about this. They simply don’t.