Powershell – Exchange 2010 powershell – select custom attributes

What you want to do is create a custom Powershell module. Start here for the official MS documentation, but there are plenty of other tutorials available online. There is also a book called Professional Windows Powershell Programming that covers development of custom cmdlets and modules. It dives deep into the .NET level. Haven't read it myself so can't make a recommendation.

First, the ForwardingAddress property does not have to correspond to a Mail Contact. (I have several that list other mailboxes and quite a few that list distribution groups). Just remember that as we continue.

The problem is your use of the SELECT verb and how it is returning a portion of the property instead of the full object. Try: Remove-MailContact $(Get-Mailbox "jsmith").ForwardingAddress

This is getting the object that is the forwarding address. Another alternative (if you don't mind multi-lining it) would be:

$mailbox = Get-Mailbox "jsmith"
$forward = $mailbox.ForwardingAddress
Remove-MailContact $forward

Remember that all of this assumes the ForwardingAddress is referencing an object of type MailContact, which isn't guaranteed (it can be any type of Direcotry.ADObjectId)