How to hide users from the GAL in Office 365 synchronized from on-premises
Hiding users from the Global Address List (GAL) is fairly straightforward when the user is a cloud account. Simply “Hide from address list” from the Exchange Online console or run some quick PowerShell: $LiveCred = Get-Credential $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection Import-PSSession $Session Set-Mailbox -Identity [email protected] -HiddenFromAddressListsEnabled $true