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?
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?