After many years and experience with most of the major products in SPAM filtering, I would say you are not going to find a solution to your problem. in-house SPAM filters will definitely mark them as spam but still deliver them to your inbox or junk folder depending on the configuration. Its risky to set a policy to mark SPAM emails to be deleted by default. Its impossible to for an administrator to monitor and release emails. The solution I found is to use a hosted mail security service like MX Logic (by McAfee). I use mimecast. Instead of the 600+ junk mails my boss used to recieve in a day; he now recieves a maximum of 2 or 3 junk emails per week. If there is difficulty in identifying a particular email, they will send emails with subject and from address to the reciepient who can decide whether to block, permit or release the email. These services are pretty cheap too and offers disaster recovery, business continuity and archiving in a wholesome package. We dissolved our microsoft exchange clustered mail servers which were using 6 high end servers to just one!!! if our server fails, users will be able to send and recoeve emails using the hosted site. when our server comes back up online, they will deliver all emails to exchange. the reports, security, control and monitoring is GREATER than what Exchange offers.
something scary about 3rd party having access to my emails. From a security pespective, I'd rather maintain my owwn email system.
I USED TO think like you :-) The risk is more when you host it inside, if you ask me. These companies are SAS70 audited. Though its not silverbullet, it provides a certain level of assurance. Then the cost is cheaper when you consider all the benefits.