Using classic Outlook for Windows in business environments
I managed to block it on Firefox using Element picker mode on uBlock Origin. Just select the banner that the ad appears on at the top of your emails (as if it's an email itself) with the element picker, then choose the top option in the choices that come up, as per the attached screenshot. It's working for me and has done for a while now but no doubt Microsoft will eventually come up with a way that negates this.