Possible ways to prevent email spoofing at MIME level

Need your help. Recently we are getting so many SPAM emails to sent from our own email address. We have GFI Mailessentials deployed an Edge Transport server. But GFI does block spoofing at the SMTP level and doesn't block spoofing at the MIME level. Could you recommend the possible ways to prevent email spoofing at the MIME level? Thanks in Advance !!!

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Rajakumar