Hide AD synced users from GAL in O365
Only if msExchHideFromAddressLists is missing from: AD > User > Properties > Attribute Editor
Start > Synchronization Rules Editor
In from AD – User join > Edit > Yes
Precedence: 51 (or higher, up to 99)
Transformations > Add transformation
FlowType: Expression
Target Attribute: msExchHideFromAddressLists
Source:
Save > Yes > OK
PowerShell:
AD > User > Properties > Attribute Editor > msExchAssistantName
PowerShell: