https://mailbox.org/en/post/warning-new-outlook-sends-passwords-mails-and-other-data-to-microsoft?nl=emailbox.org

"Microsoft steals access data" - When the well-known German IT portal "Heise Online" uses such drastic words in its headline, then something is up. If Microsoft has its way, all Windows users will have to switch to the latest version of Microsoft Outlook. But: Not only can the IMAP and SMTP access data of your e-mail account be transferred to Microsoft, but all e-mails in the INBOX can also be copied to the Microsoft servers, even if you have your mailbox with a completely different provider such as mailbox.org.

If you set up a new account in the software, Microsoft offers a supposed security function: It says that non-Microsoft accounts are synchronised with the Microsoft cloud and that copies of "emails, calendars and contacts are therefore synchronised between your email provider and Microsoft data centres".