If you have your own email server like me, you may need to know something about SPF record.
From the SPF website, I got the following description:
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.
Use DNS Stuff‘s DNS report tool, you can see if your email server has a SPF record.
To add a SPF record in Windows server.
I go to http://www.michaelbrumm.com/spfwindowsdns/
Update: The site with mentioned link is offline. Here is an archive page.
Follow the instruction to setup my DNS server in Windows 2003
If you can, please set it up to protect your domain name.