Block external Email (to me from me) on exchange 2007

We get lots of spam lately (asually viagra, amazon etc) that have one of our exchange users as recipient and sender.  Is there an easy way to block external mail that says it comes from our domain and not stop internal traffic from our domain.
All of our users are either local or use webmail no SMTP users.

Thanks
FPCSAsked:
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ans-ansdenverConnect With a Mentor Commented:
See here:
http://technet.microsoft.com/en-us/library/bb124739.aspx
It covers the Exchange 2007 content filtering (2007 version of IMF)
Check out the sender filtering section, sounds like you are being spoofed. Check of the message header will tell you the IP address of the sending email server.

And, even though you have no clients that pop/smtp email your exchange server uses smtp to connect to other email servers and allow connections to receive smtp email.
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