SPF
http://old.openspf.org/index.html is a great little site. To quote the site:
Cross posted from my blog at http://schema.sol3.net/kbarrowsSPF is Sender Policy Framework
SPF fights return-path address forgery and makes it easier to identify spoofs.
Domain owners identify sending mail servers in DNS.
SMTP receivers verify the envelope sender address against this information, and can distinguish authentic messages from forgeries before any message data is transmitted.