Sending Mail from Horde Problem

[Martin Breuer]

Hi

We run two servers, one with Horde and one with Kolab2.
If we send our mails with an email client like thunderbird, we have no 
problems.
But if we use IMP (Horde), we receive errors and the following entry in 
the postfix log file:

192.168.1.241 is our Horde webserver

    <info> postfix/smtpd[16566]: NOQUEUE: reject: RCPT from
    unknown[192.168.1.241]: 554 <tglaeser at netzeteam.de>: Sender address
    rejected: Invalid sender; from=<tglaeser at netzeteam.de>
    to=<tglaeser at netzeteam.de> proto=ESMTP helo=<localhost>

There must be another way than the setup of privileged networks the get 
IMP working.
Does someone have a solution or a idea?

Best Regards
Martin