Block email faking to be from our domain but coming from outside?

[Remco Barendse]

See more and more messages incoming with fraud attempts. The mail is 
constructed to look like from someone in our organization sent it and is 
addressed to people within the organization.

Is there any way to block email with a sender that pretends to be 
coming from @myowndomain.com but coming from outside?

I use Exchange and all real email is coming only from Exchange, never from 
outside.

What would be the right way to do it?



Also, some companies sign incoming email messages with a one liner as the 
very first line of an email like :
"THIS EMAIL ORIGINATED FROM OUTSIDE OUR ORGANIZATION"

How to do that ? I found that MailScanner can sign messages but only at 
the bottom of an email?