Our public IP address is on a blocklist. Does that mean we have been hacked?
Not necessarily, but it means something behind that address behaved badly, or was reported as doing so. Common causes are a compromised mailbox or device sending spam, a computer infected with malware that belongs to a botnet, an open service abused by others, or an address that belonged to someone else before your provider assigned it to you. Look up which list it is and the reason it gives, check your mail server and firewall logs for unusual outbound mail or connections, and change the password of any mailbox that sent unexpected messages. Ask for removal only after the cause is fixed, or you will be listed again. A blocklist points to your public address, which many devices share; NetSenX flags beaconing and unusual outbound traffic per host, on every plan, which helps you find the device.
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