If you’ve migrated users to Office 365 (exchange online) and users get prompted for a password when opening outlook you more than likely need to enable Modern Authentication for Exchange Online (https://social.technet.microsoft.com/wiki/contents/articles/32711.exchange-online-how-to-enable-your-tenant-for-modern-authentication.aspx)
Open a Powershell prompt and connect to Exchange online by the following steps.
$UserCredential = Get-Credential
Enter user credentials for office 365 *@*.onmicrosoft.com account
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
As long as you didn’t do this on an Exchange server you see it connect to Exchange Online and successfully export lots of commands.
Get-OrganizationConfig | ft name,*Oauth*
You will most likley see it as FALSE, enable MOdern Authenication for Exchange with the following command;
Now logon to your ADFS server, open powershell and check the following;
Enable-AdfsEndpoint -TargetAddressPath “/adfs/services/trust/13/windowstransport”
Repeat Restart-Service adfssrv on every ADFS server in the farm