You can stop Email Harvesting in Smartermail by going to SECURITY > ADVANCED SETTINGS > ABUSE DETECTION. If you do not have any Rules listed, then click on the [NEW] button. For "Detection Type" select "Bad SMTP Sessions (Harvesting)". You can then set your "Time Frame", "Bad Sessions Before Block" and "Time to Block".
I have two Bad SMTP Sessions (Harvesting) set... one with a Time Frame of 5 minutes, 5 Bad Sessions, and a 60 min Time to Block...the other with a Time Frame of 60 minutes, 15 Bad Sessions, and a 43200 min Time to Block.