I can't believe how this guys manage the support of this product.. They show up but don't answer any of the questions of the people with the real problem, so they quote them and still missing...
All the people who have this problem, please bump this thread up till we have a proper answer, every day we have to run the query to erease the guest customer so the clients can access to their sites.. it's so embarrassing..
Please refer to two forum posts (one of my own) where I managed to find a solution for this issue.
Check your RAW IIS logs to see what is causing the exploit. If it's guest users sending emails set the following to false in your settings: