I have had similar things happen with datnet and a couple other sites that I have visited in the past months.

Programs called Spambots crawl the web and search for the @ symbol that is associated with all of our addresses. If you have ever posted your email address on this site, then it is quite probable that a program like this has picked it up. From their, the program can also grab your username....there isn't a lot that can be done to stop it.

Once the Spambots have the addresses, automated tools can be used to send the mail out. I am guessing that the attachment was a text file xxxx.txt? It is the easiest to send because spam filters struggle to pick them up as spam mail.

Like Dodgeman said, if it looks dodgy, delete it. Its not worth the risk..