If one spammer is abusing your system, it's a sure bet that other spammers will find it (if by nothing else, looking at the origin of this spam and finding the machine YOUR domain is on is an open relay, or has an abusable script as Fahim suggested) and start abusing it too. This is the kind of thing (abuse/overuse of Internet systems by spammers) that results in systems crashing and loss of actual email you wanted to receive, and it's just one of many things that makes spammers SO BAD...
Not to mention some overeager/lazy sysadmins at some webhosts simply blocking scripts/folders instead of actually figuring out where the issue is
I've been on that end of things where sometimes you have hundreds of mails going out per second/minute and destabilizing the server. Usually, we investigate, find the security hole and disable just that script. But sometimes, people simply block everything and blame the owner of the account. Not saying that this is the case in this instance, but rather, explaining things a bit more in detail since I think I'm going to sticky this thread once the issue is resolved
Additionally, I know that ben already posted a link on how to get your e-mail headers and it's very comprehensive. But for the sake of posterity (in case that link goes AWOL) and for clarity, I'm copying and pasting the important bits from there and adding a few changes for clarity:
How to view your mail headers
Elm, Pine, and Mutt
Press "h" from the message selection menu to view the full headers of the currently selected message.
Eudora
Open the message. Under the title bar are four options. The second from the left is a box which says "Blah, Blah, Blah." Click on that to display the full headers.
Hotmail
Go into "Options", "Preferences", and choose "Message headers". You'll want to choose the "Full" option to display Received: headers. "Advanced" will display that as well as MIME headers. Do note, however, that sometimes Hotmail has to press some previous generation mailservers into service, and messages sent through those mailservers won't show any headers no matter what. :-(
Lotus Notes 4.6.x
Open the offending mail. Click on "Actions", then "Delivery information". Cut and paste the text from the bottom box, marked "Delivery information:".
Netscape Mail
Choose "OPTIONS" from the options menu bar. Listed as an option is "Show Headers". Choose full headers.
Outlook Express
Open the message. Choose "File" from the options menu bar. Listed as an option is "properties". Another window will open, showing two tabs. You want to choose the one titled "Details". Then cut and paste the headers into the message you want to forward.
Outlook 2000/XP
Double click on the message to open it up, click on "View --> Options", and you will see the message headers in a box at the bottom of the window. You can copy/paste them from that window.
Outlook 2007
Select the message on your message list and right-click on it to get a context menu. Select "Message Options". The dialog which opens up witll have the headers listed at the bottom under "Internet Headers". You can copy/paste them from that window.
Pegasus
Choose "READER" from the options menu bar. Listed as an option is: "Show all Headers". This does not work for HTML messages, however. A workaround is to select the message properties, and de-selecting "Contains HTML data".
ThunderBird
Select View - Headers - All from the main menu. Now when you view a message (either in the preview pane or in a new window) it will display the full headers for the message. Unfortunately, it does not allow you to select all the headers. You can only select individual lines and copy them. So a screenshot might be your best option here.