For SpamExperts Hosted Cloud the hostname is delivery.antispamcloud.com (IP addresses may change, hence only hostname based Allow listing is supported).
Please make sure your firewall does not block DNS port 53 TCP (to avoid hitting the UDP packet limit size).
Alternatively if allowing emails based on hostname is not an option, you could change your destination mail server to listen on an alternative one, such as port 2525 instead of the default 25 . The special port can be set for delivery when editing the destination route for your domain.