After some researching, it turns out that most cloud servers' IP's (if not all) are by default "blocklisted" by Spamhaus. I am pretty upset that there is such thing and it is not more widely known. Their reason is to prevent spammers from using cloud servers, given that cloud servers can be easily obtained in volume. At least Spamhaus makes it fairly easy to look-up/remove your blocked IP's.
Solution: Go to