Sure, I can enumerate the handful of the Fortune 500 that don't use Microsoft. Palo Alto Networks, for example, has TAP sitting in front of Google. In PANW's case it's because of a broader partnership Nikesh put together with Google in 2018, which also involved moving from AWS to GCP. This is stupendously uncommon, though.
If you were to look through the System -> Inbound Mail settings for every PPS customer, you'd find a sea of x.mail.protection.outlook.com, some on-prem Exchange servers, and practically nothing else. I'm comfortable with "always" as a description of this state of affairs, but you do you.
