The situation that you describe, in which the From field of messages claims to be from users on your network, is known as "spoofing." This is a tactic commonly used by spammers. Most anti-spam services, whether outsourced or hosted internally, are sophisticated enough to recognize spoof attempts and will block these messages. You can also setup a SPF record for your domain name that identifies which servers on the Internet are authorized to send on your behalf.
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