I'm testing MxGuardDog right now. Over the past day it quarantined 82 messages. There were 8 false positives from legitimate marketing companies. Ten percent is a poor false positive rate. I guess as long as you put time in to maintain the quarantine queue, the service is usable. Usable for a small group, but would probably upset individuals on a server with a large number of email users. Examples of false-positives include: Art.com, AMCtheatres.com, Quora.com, Janus.com, and Shutterfly, Joann Fabrics. These are all big name companies whose emails may be missed by some customers who subscribe to them.