Add the capability of Zimbra Mail Server use Spam Online for improvement Zimbra Mail Server. Two anti spam online :

1. b.barracudacentral.org
2. zen.spamhaus.org
   

The following is a way to add anti-spam online at Zimbra Mail Server :

1. Open Zimbra Admin
2. Go to the Configuration | Global Settings | MTA
3. Add the column name List Of RBLs anti-spam online (b.barracudacentral.org & zen.spamhaus.org)
4. Save

The Following Video Tutorial  : Improvement : Add Anti Spam Online on Zimbra Mail Server