Using Outlook for iOS in business environments for email and scheduling
Hi Ruthie,
I'm Diane and I'm happy to help you today.
Are you getting this error in the Outlook mobile app? If so, report it though the app. They can check to see if it's a bug in the app or with the service. Get in-app help for Outlook for iOS and Android https://support.microsoft.com/office/218a22d1-9fa5-4889-b689-de1c63493243
It's not an alias issue - as long as the primary address is a Microsoft address, it should work.
I'm not sure if Outlook on the phone goes through Apple's Cloud?
No, it does not. And slowness syncing iCloud would be an Apple issue.
Let me know how it goes, I'm here to help you further if needed.
-- Diane