Well looks like its working. just make sure you have them set their email clients to use port 2525.
Code:
david@david:~$ telnet www.dcs-online.nl 2525
Trying 84.31.236.18...
Connected to dcs-online.nl.
Escape character is '^]'.
220 server1.dcs-online.nl ESMTP Postfix (Ubuntu)
ehlo server1.doc-online.nl
250-server1.dcs-online.nl
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-AUTH PLAIN LOGIN
250-AUTH=PLAIN LOGIN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
quit
221 2.0.0 Bye
Connection closed by foreign host.