@sangroot this unfortunately means that your server got an IP with a bad reputation ending up in the blocklist. To explain, those IPs are constantly reused and it could just be that either this IP itself was used by a spammer in the past or that some other IPs in that IP block might have been spammer IPs.
This is not unusual though and it takes a bit to get IP reputation up and manually removing it from blacklists by appealing is the first step in this case.
If you need to deliver emails reliably from the start, you may consider using a mail relay configured through Cloudron for some time until you are off of all those blocklists.