I need to call upon the exchange experts out here as I'm not one and I don't know what else to check.
Some of our users email a particular domain and have other internal users in the cc field. When the person on the external domain replies all we see that one of our internal users that was in the cc field has changed from name@domain.com to name@domain.invalid. Hence the user that had their email address changed does not get the email. It's not just happening to the same internal user in the cc field either, I have seen it happen to another as well.
I have checked that the user sending the email is putting the correct email address in the first place and not some cached address that may be wrong. I have also checked that the original email does get delivered to the person in the cc field.
It's only once the external person replies it...