I've installed Zimbra 6 and frequently thousand messages are queued in name of another host not configured in Zimbra. Example: my domain name is mydomain.com, but I have thousand messages in queue from domain spamdomain.com, and the server are sent this messages. I search in /data/postfix/ and find the queued information about this messages, and this messages are coming from a external IP.

There is some configuration in zimbra or postfix to block this problem? I used the tool Email Server Test - Online SMTP diagnostics tool - MxToolbox to test the server, and the result is that the server is not a open relay.