Office 365 - hiding an external contact from the GAL

1. Connect to Exchange Online by using PowerShell. For more information about how to do this, see the following article:
http://help.outlook.com/en-us/beta/cc952755.aspx?sl=1

2. Run the following cmdlet to hide an external mail contact:

Set-MailContact -Identity -HiddenFromAddressListsEnabled $true

By doing so, the external mail contact will be hidden from global address list (GAL).