Vista - Clean WindowsEmailAddress user value

Hi, I have written these simple script lines in PowerShell:

##
$User = "0743159"
$excUser = get-mailbox -identity $User
$excPrimarySmtpAddress = $excUser.PrimarySmtpAddress
Set-User -identity $User -WindowsEmailAddress ""
##

How can I do to clean WindowsEmailAddress value ? I have found in Internet
two methods: using PQuest tools and another using vbscript, but I would do
not like to use of these methods. I would like to use PowerShell method.
Thank you very much for your support. I wait your answer.

You were already told 3 days ago that you are in the wrong
group. This group is for VBScript. PowerShell is not VBScript.
The group for PowerShell is here:

microsoft.public.windows.powershell