New Mail Alerts are also created by Outlook, at least from Outlook 2010 onward, though which versions have it on versus off by default I cannot say.
I can say that if you open Outlook help, and search on "new mail alert" you will get step by step instructions for configuring all aspects of that feature, including turning it off entirely.
Brian - Windows 10 Home, 64-Bit, Version 1803, Build 17134
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back