So they are saying they are getting brute force attempts from your server to another server? Depending on how it happened it might be hard to track down. You could go through logs in /var/log but I doubt it would come up with anything. Either you have an affected site on your server or an abusive user. If it was from any of the websites you were hosting the logs would be in /var/log/httpd, but if you are not familiar with what you are looking for your best bet would be to hire an administration company that can go through your server and help you determine what caused the problem.