Had a play with SPF (Sender Policy Framework) today. Enabled outbound SPF by creting the following DNS record for my domain (thefoleyhouse.co.uk) at http://www.freeparking.co.uk.
TXT "thefoleyhouse.co.uk" "v=spf1 a:smtp.ntlworld.com a:cartman.homedns.org mx -all"
This means that for "thefoleyhouse.co.uk" only smtp.ntlworld.com (my ISP relay server), cartman.homedns.org (my exchange box) and and servers marked in my mx records are allowed to send mail from my domain.
This is of course only enforced by receiving smtp servers that also use SPF. Now if only I could get plus.net to create this for my work domain we would be seeing a reduction in domain spoofing.