Exchange 2010: Spoofing issue

Today, a couple users received a spam email from another user within the company.  The email originated from an IP address in latin america, not from within our network.  To me this means that someone through data mining etc, got a few user email addresses and sent an email to multiple people using one of the addresses as the sender.

What can I do to prevent this from happening again?
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VoodooFrogAuthor Commented:
We already have spf records set up. And Sender ID is already enabled.
AmitIT ArchitectCommented:
You can implement 3rd party spam solution like MailMarshal or Trend InterScan.
Postini is another good one.
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