Microsoft’s plan is to “motivate” users to choose the Outlook client’s Focused Inbox option in greater numbers, which removes all ads in exchange for paying a monthly subscription fee.
Acting on the principle “if you can’t make something better, then at least make it look worse,” Microsoft will “pollute” the Inbox by showing users free versions of the Outlook client with even more ads, thus highlighting the benefits of the Focused option, which separates the Inbox into two tabs. The primary tab shows all the messages you’re interested in and the secondary tab shows those suspected to be in the unsolicited category. Conveniently, this is also where the ads that Microsoft uses to annoy users of the free Outlook client appear.
The change has been phased in over the past few months and essentially means that Microsoft is intentionally making it harder for “free” users to avoid the ads displayed in the mobile version of the Outlook client. Whichever inbox mode you use, ads will appear somewhere in the mix – either at the top of the Other tab or at the top of your regular inbox. Really, the only way to avoid ads in mobile Outlook now is to pay for a Microsoft 365 subscription.