Once unsolicited email reaches the server, it must store, process, and forward the messages.  Viruses can attack the mail server and ingenious messages can trick your mail server into telling the sender what valid email addresses exist in you office.  Some messages can turn your mail server into a “slave server,” causing your server to send thousands of messages to other internet users.  This relaying devastates your bandwidth further, damages your company’s presence on the internet and even causes your domain to become blacklisted.  Once blacklisted, real messages from your company will be denied delivery, and it is very difficult to be removed from internet blacklists.