I have read in this wiki article: Improving Anti-spam system - Zimbra :: Wiki
That if a spamassasin score is high enough, the message in question is automatically deleted.
I tested this by sending the GTUBE spam test message to this server, and as expected it did not arrive.
I am hoping to find a way to prevent this auto deletion - I just want spam to go into the junk mail folder.
Thanks for any suggestions!