Well looks like its working. just make sure you have them set their email clients to use port 2525.
david@david:~$ telnet www.dcs-online.nl 2525
Connected to dcs-online.nl.
Escape character is '^]'.
220 server1.dcs-online.nl ESMTP Postfix (Ubuntu)
250-AUTH PLAIN LOGIN
221 2.0.0 Bye
Connection closed by foreign host.