[Exim] expansion failed in auth_login

Hello,

I try to send my mail from my local EXIM-MTA by smtp to GMX for separate
users in my local net.

The different GMX-accounts should be choosen my comparing the $local_part of
the mail.

This is my config:
if the user name is "hippi", then 5647568 and xyzyoyo should be used for
authentication at GMX
######################################################################
#                   AUTHENTICATOR CONFIGURATION                      #
######################################################################
auth_login:
    driver = plaintext
    public_name = LOGIN
    client_send = ${if match{$local_part}{hippi}{:5647568:xyzyoyo}fail}

But, it doesn't work.

I get an error-message: missimg or misplaced { or }.

I hope there is someone who could help me to solve my problem.

Thank You.

Danny Herrlitz