What is an open proxy?
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An open proxy is a computer or server that strangers can send their traffic through, usually because it was left open or set up wrong, so their spam goes out looking like it came from you. It is one of the things that gets an IP onto a blocklist, and it is a security problem, not a list problem. The blocklists group open proxies with compromised machines: infected devices, spambots, hijacked routers and hijacked systems used for spam delivery. The common kinds are HTTP and SOCKS proxies.
Several blocklists list them on purpose. Spamhaus's XBL lists IPs of compromised machines, open proxies included. The CBL, which feeds the XBL and ZEN, lists IPs that show signs of compromise, open proxies among them. SORBS keeps zones just for open proxy servers, DroneBL targets them, and Barracuda flags spam proxies automatically.
If you are listed for one, the listing isn't about your content or your list hygiene. Check whether your server is acting as an open proxy (HTTP, SOCKS, or similar), and fix the security issue before you ask for removal. Spamhaus XBL removals are largely automated once it is fixed, and most SORBS zones clear on their own once the underlying issue is resolved.
SORBS names "running an open relay or proxy" side by side as a common trigger. For the mail server version, see What is an open relay?
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