[BlueOnyx:16728] Strange Emails

[tom]

This morning I noticed strange emails received from no sender to no 
recipient, with no body or header. The only thing that comes in is [!!SPAM]
in the subject. So far I have received 100s of them at about 3 every 5 
minutes. I uninstalled and reinstalled my DNSBL's. I also re-booted the 
system but I still continue to receive them.
Any ideas?
Thanks in advance
Tom