Forwarding eMails from Exchange to Icewarp brakes, but not via distribution list

Hello,

we are testing heavily Icvewarp as replacement for our outdated Exchange.

Unfortunatly there is nearly no docu around in the internet, which makes it not easy :-(

We have now set up the Icewarp together with the Exchange.

All mails coming to the Exchange postboxes are forwarded via a user-setting to the Icewarp server.

Icewarp server uses Exchange as mail-relay.

Therefore all incoming eMails should get received by both servers and all sending is done via Exchange.

But unfortunatly this only happens with half of the mails.

If someone send a eMail from outside or inside to a distribution list on the Exchange server, the eMail is received by all members of that distribution group in Exchange AND in Icewarp. Which is fine and as expected.

BUT if someone send a eMail to a Exchange user, the Icewarp server blocks the message creating a error-message in Exchange AND Icewarp saying:

Undiliverable (Unzustellbar)

icewarp.iteanova.com hat Ihre Nachricht an die folgende E-Mail-Adresse zurückgewiesen:

Myuser@icewarp.mydom.com


icewarp.iteanova.com hat diesen Fehler ausgegeben:
<Myuser@icewarp.mydom.com>... Permission denied


Ihre Nachricht wurde 
aufgrund eines Berechtigungs- oder Sicherheitsproblems nicht zugestellt.
 Möglicherweise wurde sie von einem Moderator zurückgewiesen, die 
Adresse akzeptiert nur E-Mails von bestimmten Absendern oder die 
Übermittlung wurde durch eine andere Einschränkung verhindert.

Environment:

• Exchange with domaine Mydom.com and DNS exchange.mydom.com
• Icewarp with domaine Mydom.com and DNS icewarp.mydom.com.
• In Exchange for every user a additional mail-contact pointing to the eMail-address user@icewarp.mydom.com
• In Exchange a forwarding for every user to his mail-contact pointing at user@icewarp.mydom.com
• Exchange configured as mail-relay for icewarp. Icewarp configured to use Exchange as mail-realy.
• MX-record mydom.com pointing to exchange.mydom.com
• MX-record icewarp.mydom.com pointing to icewarp.mydom.com
• In Icewarp, Exchange is listed as "Trusted Host"

We need to solve this problem urgently as our testing is stopped and more and more colleauges began supporting sticking on Exchange, which i really don't like to :-(

Thanks for any help.