@uxbod
Thank you

and thanks for that link. I checked it out but the first commando is already giving me an error. Since we don't have postconf on the server installed.
@gnyce
Actually we do still have access to the old domain. Problem is that we want to let it die out and therefore the alias isn't really an option because when we dump the old domain there will be no more forwarding.
Sincerely,
Christophe