Sender/Recipient Filtering: Blocking spam email that apears to come from another adress on my exchange server.

I used to get lots of spam but have since implemented "Choicemail" to block all my spam.  It was about 99.9% effective.
Starting a couple weeks ago, I get about 10 spam emails a day that appear to come from another user in my orginization (employee@mydomain.com).  Is there a way to set up filtering so that any email that comes from an address within my organization but does not come from within my own exchange server is blocked?  I do use Outlook Web Access, but, the way I understand it, the actuall email is still sent from my exchange server.  OWA is just a way of accessing it remotely.  
Please advise.
Mike_PlugAsked:
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dmessmanConnect With a Mentor Commented:
What you need is an SPF record and then your anti-spam system to check against that SPF record.

An SPF (sender policy framework) record defines what servers are authorized to send mail from your domain.  You'd want to make an SPF record here:

http://www.openspf.org/

And then put that SPF record in your DNS records on your nameserver and then make sure your anti-spam system is checking against that SPF record.
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