Hello,
Yes, this is all technically possible, but the specifics of a solution depend on what email infrastructure you have today.
Also, keep in mind that it would only work for the mail servers you control.
For example your user could login to a free webmail and still send To: the destination user to get around your monitoring.
BTW, I think your approach could be improved. Check with your company lawyers but it seems more correct to archive a copy of all outgoing email, and then your HR dept can approve the user's manager checking the user's outbound mail flows on request.
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