I'm asked for solving this particular case in a zimbra 6 environment: Is it possible to prevent user from sending an email if sent to a specified addresses or domains at the same time?

I'm me@domainA.com, I have a customer@domainB.com and a competitor@domainC.com. Is there any way to prevent domainA.com users from send an email to customer@domainB.com and to competitor@domainC.com as CC or BCC?

Thank you