Hi there,
We have a client who was using Azure AD sync to connect their O365 tenancy to a local domain controller. The DC has long since gone and they have been cloud only for several years now. The Set-MsolDirSyncEnabled flag has been disabled in O365 Azure to say it's no longer linked.
On the whole, everything is working fine except when users sign into Outlook for the first time where they are prompted to login with the username "OLDDOMAIN\email@company.com". This is confusing for the users and is breaking the policy which should be silently signing users into Outlook automatically.
Does anyone know how to fix this? I thought it would be something going on in Azure AD but now I'm wondering if it might be an Exchange Online issue?