SpamExperts "Hosted Cloud" will try to automatically determine the destination route for your domain's mail. Be sure to check for accuracy, as part of your initial configuration.
To prevent spammers from delivering spam directly to your mail server(s) without filtering, you have to make sure your mail server only accepts emails originating from the Hosted Cloud filtering system.
For redundancy reasons, Hosted Cloud nodes are located in many different datacenters. Therefore they do not share a single IP range and since the Hosted Cloud is regularly expanded, new IP addresses may be added at any time.
To only accept messages from the Hosted Cloud filtering nodes, you need to allow emails based on SpamExperts' delivery hostname "delivery.antispamcloud.com". This hostname contains all active delivery IP addresses. Alternatively you can allow traffic from any IP address with the PTR record *.antispamcloud.com.
Make sure your firewall does not block port 53 TCP.
Alternatively, if allowing emails based on hostname is not an option, you could change your destination mail server to listen on an alternative port, such as 2525 instead of the default 25 . The special port can be set for delivery when editing the destination route for your domain.
For additional details: https://my.spamexperts.com/knowledgebase.php?language=english&action=displayarticle&id=31