Adding Custom SMTP address in Bulk Exchange 2010

Hi,

I'm trying to create a script that gets all user mailboxes in Exchange 2010, then adds an additional custom SMTP address (SIP) without removing any existing email address.  If I run the script for one user, it works fine (below)

This works for an individual user:

$SIPUsers = Get-Mailbox -Identity user

ForEach ($user in $SIPUsers)
{
    $SIPUsers.EmailAddresses += "SIP:" + $user.SamAccountName + "@domain.org"
    Set-Mailbox -Identity $user -EmailAddresses $SIPUsers.EmailAddresses
}

When I run this script (below), it removes all SMTP address, x500, custom secondary alias etc and applies the default email policy.  I do get an error as well, which is:

$SIPUsers = Get-Mailbox -RecipientTypeDetails UserMailbox -ResultSize unlimited

ForEach ($user in $SIPUsers)
{
    $SIPUsers.EmailAddresses += 'SIP:' + $user.SamAccountName +'@ynhh.sso.ynhh.org'
    Set-Mailbox -Identity $user -EmailAddresses $SIPUsers.EmailAddresses
}

Error:

Property 'EmailAddresses' cannot be found on this object; make sure it exists and is settable.
At C:\Scripts\SIPAdd.ps1:5 char:15
+     $SIPUsers. <<<< EmailAddresses += "SIP:" + $user.SamAccountName + "@ynhh.sso.ynhh.org"
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : PropertyAssignmentException