Well, I tried to relay mail through your server, and that doesn't work, so your server itself is not an open relay. Does your server act as an outgoing mail server for other servers/desktops in your LAN/network, or is there a web application on your mailserver? I see a lot of connections from localhost, so there may be a web application being abused by external parties (think of webforms, formmail, php forms, guestbooks, etc.). Though usually spam through a webserver has something like www@localhost or
wwwrun@your.server.com as the sender address.