Monarch client security became an issue last year when a German website reported some issues. It turns out that the reported problems are mostly hyperbole, but that hasn’t stopped them persisting, especially when email client competitors like Proton weigh in. It’s regrettable that much of the commentary is based on an incomplete understanding of how Monarch works, but Microsoft doesn’t help themselves by not explaining the facts.
https://office365itpros.com/2024/04/11/monarch-client-security/
