You can use the Autodiscover function of Microsoft Exchange outside the forest.
And here is how to do it with PowerShell.
PowerShell commands to use Exchange Autodiscover
Maybe you want to use the Autodiscover function in a trusted ressource forest. The Autodiscover information has to be synchronized out of the Active Directory Configuration Partition.
Microsoft offers a PowerShell command for that:
$mycredentials = Get-Credential
the following information is written once:
Export-AutoDiscoverConfig -DomainController <SourceDC> -TargetForestDomainController <TargetDC> -TargetForestCredential $mycredentials -MultipleExchangeDeployments $true
Importance of Autodiscover
The Autodiscover function is crucial for Outlook 2007/2010.
Without it the Out of Office Assistant cannot be executed.
The download of the offline address book would fail as well.