[exim] Sending mails to some domains won't work

Hi all,
I just set up a new system, and got the following issue: Sending to the most of recipients work, but some like users in gmx.de won’t accept my mails. I’m running Debian 3.1 with confixx.
2005-05-19 19:36:11 H=(PC) [83.135.xxx.2xxx] F=<someone@???> rejected RCPT <someone@???>: relay not permitted
Like I said, the same mail-account can send to other recipients…

Thanks in advance, Chris

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