Tom,

The correct way to handle this situation is to use SPF. Check http://xeams.com/spf.htm for details. I recommend you put the following line as a TXT record in your DNS server:

v=spf1 a mx -all

Once that is done, Xeams will automatically assign a score if it detects a forgery. Besides SPF, we also recommend using DKIM and DMARC. Check http://xeams.com/dkim.htm and http://xeams.com/dmarc.htm for details.