If you try this:
$ol = New-Object -comObject Outlook.Application
$mail = $ol.CreateItem(0)
$mail | Get-Member
You will get a list of all the methods / properties available on the mail object.
One property is DeferredDeliveryTime. You can install it like this:
$mail.DeferredDeliveryTime = "11/2/2013 10:50:00 AM"
Or:
$date = Get-Date
$date = $date.AddMinutes(10)
$mail.DeferredDeliveryTime = $date
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