How to disable relaying in Exim?

Hi,

I am getting blacklisted due to emails being relayed through my server. I know for a fact that the user as shown in the mail headers is not doing this.

The server in question is just a web server, it is not the mail handler for any domain that is hosted on the server. The only email that is sent is by forms on the websites. (php/perl)

Hence I wish to disable relaying (except from localhost) and/or pop-before-smtp on this machine, can somebody advise how I can do this or suggest any ideas if I am going about it wrong?

Thanks in advance.

-- Ashish