One way to get around this could be to give him 2 email addresses: the ceoPrivate and ceoBlackhole. If you make the ceoBlackhole the default SMTP address in his user account in AD, then this will be the “sender address” as far as the recipients are concerned. Some mail systems check the senders email domain for existence, so don’t use ceoBlackhole@nonexistingdomain.com, but use something you control. If you choose to give the CEO an email from a subdomain and to keep the “obvious CEO email” unused, then you can configure an event sink that does the “catch all”-trick – else Exchange will drop messages that cannot be delivered. Also keep his email away from websites as you can search for this in Google.