Protection against fake mails

["WALI" <hkhasgiwale () gmail com>]

If we a do a google search for the following string "Send free, anonymous 
and easy fake email", one of the sites we get is 'deadfake.com'...and many 
others. I can send an email to myself from myself by filling in the two 
fields at the website.

Technically, what such sites seems to be doing is that, such mailers add an 
X-Mailer attribute to the message header indicating the message origin, and 
an X-Originating-Ip (a real one).

How do I guard against such emails originating from fake email 
impersonations. Is there something I can do at our email gateway, proxy or 
exchange sever (2003) levels?