The migration from the classic Mail and Calendar app to the new Outlook app is in full swing already. Microsoft announced the deprecation of the classic apps in favor of a new Outlook app in June 2023. It introduced the new Outlook app to Insider builds a month later and announced that it would enforce the migration in early 2024.
Not all users are migrated at this point. Those who have been migrated already or installed the Outlook app directly, may notice several differences between the new Outlook app and the classic Mail app.
One of the main differences turns an ad-free email experience into one with ads. You may see ads in the inbox in the new Outlook.↫ Martin Brinkmann
Ads disguised as emails in your inbox. Microsoft will not rest until Windows resembles Times Square. What a trash fire of an operating system.