Recently, we turned on block-list spam protection provided by the zen.spamhaus.org in addition to regular DNS-based restrictions. Thanks to Zimbra’s graphing capabilities we have a powerful visual depiction of the power of Spamhaus:
This graph displays the number of messages processed by our context-based anti-spam system over a 48-hour period (from Wednesady to Friday) where each bar represents 1 hour. The green bar is the total number of messages that are not blocked (to valid or invalid users), blue is the intercepted spam count to valid users, and red is the intercepted virus count to valid users.
At 4:00 PM, I turned on zen.spamhaus.org filtering, and the effect is immediately noticeable. The overall processed message count was reduced from about 1200 to 150 per hour. A large volume of removed messages were spam sent to randomly generated usernames. This has the wonderful effect of reducing outgoing bounce messages. Also greatly blocked were both the spam sent to real users and viruses. This reduces our CPU usage since content-based search is rather intensive.
The following is a strong conclusion, but I feel it is justified by this evidence: Due to the ever increasing volume of spam, the capability for small businesses to support their own mail is now completely reliant upon the Herculean efforts of organizations like spamhaus.org.