>> your IMAP and SMTP username and password are transmitted to Microsoft in plain text.

> I think that is not true. I think that is a lie on Proton's behalf.

Sadly, it is all too true:

Microsoft lays hands on login data: Beware of the new Outlook https://www.heise.de/news/Microsoft-lays-hands-on-login-data...

Warning: New Outlook sends passwords, mails and other data to Microsoft https://mailbox.org/en/post/warning-new-outlook-sends-passwo...