BecomeMe

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