It is most likely a spam if:
1) the site mentioned is not your correct site and there is nothing in the email that correctly identifies you (your name, address, etc.)
2) the site mentioned is indeed your correct site and if on that site you have posted, or made links to, your email address (thus any visitor to the could find your email address)
In these cases you should not reply to any of them, block their email address or use filters block it, report it to your ISP and to other spam fighting agencies - if you need help in blocking email or reporting spam, let me know who your ISP and what is your mail client (outlook, yahoo webmail, etc).
If on the other hand the email has some real information about yourself and your site, and you do not recognize the sender, then I think you need to report it, in addition to the your isp and spam fighting agencies, to the Federal Communication Commission at : http://www.fcc.gov/cgb/consumers.html
Hope this helps.