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If you're sure it's an email spam attack and you can target the sender,
Report them
[spamcop.net...]
In the US
spam@uce.gov
Call their isp and complain. Repeatedly.
Configure your mail server to refuse all requests from that IP address. This is a bit drastic as it may kill legitimate requests, but if you do it for a short time they may give up and you can remove it.