The problem does not seem to be you logging into your Microsoft mail account, but logging into other mail servers:
"non-Microsoft accounts are synchronized with the Microsoft cloud"
i.e. by using outlook, you give your login data for your work/school/private mail accounts to a third party (Microsoft) which then pulls all your mail and calendars as a copy to their systems
Or: Your mail client is now a cloud app and most likely will at some point come with either a subscription or you handing over all your emails to Microsoft for advertising purposes
The problem does not seem to be you logging into your Microsoft mail account, but logging into other mail servers:
"non-Microsoft accounts are synchronized with the Microsoft cloud"
i.e. by using outlook, you give your login data for your work/school/private mail accounts to a third party (Microsoft) which then pulls all your mail and calendars as a copy to their systems
Or: Your mail client is now a cloud app and most likely will at some point come with either a subscription or you handing over all your emails to Microsoft for advertising purposes