Those ever so nice guys at Google have allowed GAFYD customers (yes the free ones included) to authenticate email with a domain key.
“Spammers can forge the ‘From’ address on mail messages so that the spam appears to come from your domain. To help prevent this, you can add a digital signature to the header of all email actually sent from your domain. Learn more”
It’s dirt easy to do if you have already added your private domain to GAFYD and allows people to verify if you actually sent them that nasty spam message (which of course you most likely didn’t… did you!?)