Did you allow delivery of any of the mail to see what it was?
It could be spam or it could be some sort of virus or trojan
penetration attempt. The end objective here may not be spam,
it might be theft of credit card numbers or the like.
If it is spam it is almost certainly hardcore fraud, like the
bogus offers for hijacked software and the advance fee frauds.
There are a bunch of hacker tools that allow a hacker to control
a constellation of hacked machines as a single unit. These
usually end up being used for criminal purposes.
Here's my theory. Well-organized spam-gangs have compromised many
thousands of personal computers all over the world. They run
a central
system somewhere that attempts to spam from one compromised
machine, and
if one transmission attempt fails, they try from the next compromised
machine, etc, until they manage to deliver (or run out of compromised
machines). It's a testament to the effectiveness of various
DNSbls that
things have gotten to the point where spammers need to put forth this
much effort to get past some people's filters.
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