Exchange 2003 Zimbra split domain mail loop

[maxvici]

Hello all,

here is my configuration :

I have an Exchange server as MX for mydomain.com.
Part of the email address of the organisation are on the Exchange server and the other part on the Zimbra.
Mail arrive to Exchange and if the recepient is not known, it follow them to the zimbra server.

Everything work well, send and receive, except that when an email address does not exist on both system a mail loop is created. What can i do to avoid that ?

Thanks a lot for your help