Как создать политику хранения по умолчанию в обмен 2010?

You can write a script and run it daily (get a list of all mailboxes without a retention policy) or use a cmdlet extension agent to detect when mailbox is created and automatically assign a rentention policy (don't have access to exchange 2010 atm so can't really test it):

<?xml version="1.0" encoding="utf-8" ?>
<Configuration version="1.0">
<Feature Name="MailboxProvisioning" Cmdlets="new-mailbox">
<ApiCall Name="OnComplete">
    if($succeeded)    {
        $mbx = get-mailbox | where{!$_.RetentionPolicy} 
        foreach($mb in $mbx){
            Set-Mailbox –RetentionPolicy "foo" 
        }
    }
</ApiCall>
</Feature>
</Configuration>

or only for this mailbox:

if($succeeded)    {
    $Name= $provisioningHandler.UserSpecifiedParameters["Name"] 
    if($provisioningHandler.UserSpecifiedParameters["RetentionPolicy"] -eq $null){ 
    Set-Mailbox -Identity $Name -RetentionPolicy "foo" 
    }
}