Monday evening I noticed something was wrong, my mail box was receiving returned mails, reason being the receiver was unknown. By the time I discovered it I had received 400 or so mails, but the total number went as high as 3000 during the evening. No, I hadn't sent 3000 mails

It turned out that the mails were sent from my Parodius Network (nesworld) account which made it even stranger as I never use that account for mail and have a forwarder to my regularly used account at a danish internet provider.
So I contacted Jeremy who runs the Parodius Network and it turned out that around 18000 mails had been sent from my account. We decided to shut down the site immediately and Jeremy went to investigate the logs.
It turns out that a hacker had discovered an exploit in a PHP script of mine and had used it to install some tools to send spam mails among other things.
At first I was a bit shocked to see the attack and as we didn't know how it had happened, I told Jeremy to just leave the site dead, but I guess he refused

He found the exploited PHP and helped out a lot to get the site back on track. It took me a few days to clean up the rest of the PHPs for possible exploits and Thursday we were finally able to bring the site back online.
I never should've "left" the plain HTML days, then this never would have happened