Exchange 2007 Userdefines Parameters in powershell

Hi @all. One more silly question just before Christmas :):
I have an Exchange 2007 installed in German.
I have to import quite a lot of external contacts which should then be member of a dynamic distribution list. The argument for the membership should be a keyword in the user defined parameters (1-15) of the contact.
my problem is, that i don't know the exact expression for the argument in the powershell. i tried the following list:
extensionAttribute1
userdefinedAttribute1
BenutzerdefiniertesAttribut1
...

The powershell command I run is this:
Import-Csv d:\import\KES-Liste1.csv | ForEach {New-MailContact -LastName $_.Nachname -Firstname $_.Vorname -Name $_.Name -DisplayName $_.Anzeigename -ExternalEmailAddress $_.ExterneEMailAdresse -Alias $_.Alias -OrganizationalUnit "pizh.local/KES" -customAttribute1 $_.BenutzerdefiniertesAttribut1 -CustomAttribute2 $_.BenutzerdefiniertesAttribut2 -CustomAttribute3 $_.BenutzerdefiniertesAttribut3}

With that I always the error:
+ FullyQualifiedErrorId : NamedParameterNotFound,Microsoft.Exchange.Management.RecipientTasks.NewMailContact

The csv-file looks like this:
Nachname,Vorname,Name,Anzeigename,ExterneEMailAdresse,Alias,BenutzerdefiniertesAttribut1,BenutzerdefiniertesAttribut2,BenutzerdefiniertesAttribut3
TestNachname,TestVorname,KES TestNachname TestVorname,KES TestNachname TestVorname,test@test.de,KESTestNachnameTestVorname,,KES,
(of course just one test argument).

Any ideas. I'll appreciate your suggestions :).