There's very little you can do about email spoofing however you can fight back with Sender Policy Framework or SPF.
The way it works is that you create a SPF record in your domain DNS zone, this record is used by email servers to identify the IP address of the server authorized to send email for your domain.
Detailed information about SPF can be found @ http://www.openspf.org , you can use their SPF Wizard to help you with the record syntax.
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