https://www.bleepingcomputer.com/news/microsoft/microsoft-is-showing-inline-ads-in-their-windows-10-mail-app-message-list/

Microsoft has a history of displaying ads in Windows 10 apps, but in the past they have typically been for their own products.  We now have reports of users seeing programmatic advertisements directly within the Windows 10 Mail app.

As can be seen below, these ads are appearing inline in their message list and could be mistaken for a new email. This may lead users to click on them without realizing they are simply an advertisement.