If legitimate emails are being sent, then please make sure the recipient email addresses were acquired using confirmed opt-in (COI), and that there is an easy method to opt-out (unsubscribe). Please also check that you have a valid PTR record for the IP, and consider setting up SPF and DKIM, if not already configured.
If you don’t have any plan to send emails at all, we are strongly recommend to close 25, 465, 587 ports in your service firewall.
By running the following script on Linux operating systems on a regular basis with CronJob, you can automatically block the IPs that are known as spammers in the spamhaus system. This script automatically Block/Unblock the ports based on the status of the spamhaus.org.
WORLDBUS support can help you to prevent unwanted spam emails if you’re in trouble situation.