Recently, one of my customers systems got hacked and the hacker "borrowed" the email server to send 27,000 spam emails from their location. This made me feel like an idiot because I set the system up and thought that we were pretty secure. Anyway, the customer bought a 3com firewall and I installed it yesterday. How can I know that we are secure and how can I learn enough to be able to assure my customers that they are safe. I feel stupid for not learning this stuff before. I am good at programming and setting up networks large and small but have absolutely no security knowledge. WHERE DO I START. I have to cover these customers.

Thank You