one of our clients is using the McAfee-Appliance and since a couple of days it is denies to accept our mails, because the IP has a „High Risk”-rating for Email, even if the server is not listed in any blacklist.
how can we get rid of this misjudgement?
There is a link on the McAfee Threat Intelligence page you linked to to submit feedback, or you can email firstname.lastname@example.org. There may be a newer address, but I don't know it. You could also have your client contact support if they truly do have a McAfee product that is blocking the message.