Help I think I am being used as a proxy forwarding server

I think my server is being used (hacked) as a proxy server, can anyone explain this and what should I do ? 84.16.252.116 - - [08/Nov/2006:11:46:14 +0000] "CONNECT mail.yahoo.com:443 HTTP/1.1" 405 317 84.16.252.116 - - [08/Nov/2006:11:47:57 +0000] "CONNECT mail.yahoo.com:443 HTTP/1.1" 405 317 84.16.252.116 - - [08/Nov/2006:11:48:10 +0000] "CONNECT mail.yahoo.com:443 HTTP/1.1" 405 317 84.16.252.116 - - [08/Nov/2006:11:48:48 +0000] "CONNECT mail.yahoo.com:443 HTTP/1.1" 405 317 84.16.252.116 - - [08/Nov/2006:12:00:14 +0000] "CONNECT mail.yahoo.com:443 HTTP/1.1" 405 317 84.16.252.116 - - [08/Nov/2006:12:23:36 +0000] "CONNECT mail.yahoo.com:443 HTTP/1.1" 405 317 84.16.252.116 - - [08/Nov/2006:12:26:09 +0000] "CONNECT mail.yahoo.com:443 HTTP/1.1" 405 317