Hi,
I am not sure what SMart Host or Anti-spam product you are using to receive mails.
In any case you got to enable Reverse DNS checks. If you have IIS SMTP then: http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/233c0fde-7315-479a-a7fb-2e9d00ff73fd.mspx?mfr=true
Hope this helps
Thanks
Nitin Gupta (gupnit)