help~ my mail server got attacked

Dear Helpers

My mail server (exim 4.52) got attacked by using random senders & random receivers.
He's also attached some viruses to emails.
I've been banned his ip address. I'm afraid that he'll use proxy or socks then do it again.

How do I protect my mail server?

pls help me..
Thank You from my heart

#a part of reject logs

2005-09-26 15:24:19 H=(xxx.com) [xxx.185.132.xxx] sender verify fail for <register@xxx.com>: no such address here
2005-09-26 15:24:19 H=(xxx.com) [xxx.185.132.xxx] F=<register@xxx.com> rejected RCPT <smith@xxx.com>: Sender verify failed
2005-09-26 15:30:07 H=(xxx.com) [xxx.185.132.xxx] sender verify fail for <administrator@xxx.com>: no such address here
2005-09-26 15:30:07 H=(xxx.com) [xxx.185.132.xxx] F=<administrator@xxx.com> rejected RCPT <ted@xxx.com>: Sender verify failed
2005-09-26 15:30:11 H=(xxx.com) [xxx.185.132.xxx] sender verify fail for <administrator@xxx.com>: no such address here