OT: extraordinary amount of spam to one domain

[Jeff Mills]

I have one domain that is consuming over 90% of the traffic to our
servers.
Yesterday we blocked 650,000 emails at the MTA and 600,000 of them were
for a single domain (we host around 50).
The ones that are getting through the MTA seem to be picked up by
MailScanner - the vast majority with a subject such as "User <random
username> special 80% OFF"
My mail logs are just a blur. If we had just one other domain doing the
same thing, I'm sure we wouldn't handle the load.
 
How can a single domain be such a target? Mind you, this is a domain
with less than 10 users.
Is it just unlucky? Is anyone else experiencing this?
 
 
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