A blacklisting by AntiCaptcha.net may indicate one of several potential issues. Being listed by AntiCaptcha.net IPv6 indicates that you likely have either sent spam, currently suffering an Open Relay or Proxy or your system has been infected by a virus/botnet.
AntiCaptcha.net IPv6 primarily focuses on reducing spam, open relay/proxies, and virus/botnet issues by having Service Providers prevent and elimate abuse on their networks. Finding yourself listed on this blacklist means you need to check your environment for one or all of those issues. If your network is clean and you are innocent of sending spam, contact your service provider to request them to investigate the cause of the abuse on their network.
AntiCaptcha.net IPv6 offers automatic delisting once they have detected your network to be clean and have not received any new sources of spam.
Anticaptcha.Net Ipv6 Reports Open Relays
Relay/Proxy-based Blacklists typically list email servers and/or hostnames that are sending Unsolicited Bulk Email (UBE) that is clearly from email addresses that are not hosted on the server or that do not match the SMTP Banner/Hostname of the Email Server, commonly referred to as an Open Relay. An Open (Mail) Relay is an SMTP server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just mail destined to or originating from known users.
Anticaptcha.Net Ipv6 Reports Sources Of Spam
Spam-based Blacklists are those that will list either single IP Addresses or entire ranges that have actually received Spam, i.e. Unsolicited Bulk Email (UBE) in their Spamtraps from an IP-Address. This could be a result of a compromised email account, an Open Relay, or simply sending mass emails / marketing and not following best practices according to the "CAN-SPAM Act of 2003" (ref: https://en.wikipedia.org/wiki/CAN-SPAM_Act_of_2003)
Anticaptcha.Net Ipv6 Reports Virus Infected Sources
Virus-based Blacklists are those that will list single IP Addresses (or hostnames) of email servers that have sent Spam traffic that is generated by some form of a Virus, Malware, Trojan, or "botnet" infection in a network. This is often a result of a user visiting a webpage that houses an infection and downloads malware on a PC which then creates a "mini SMTP" server used to hijack account information, and send bulk email to recipients in the Users' email address book. It can affect the actual machine housing the "Email server platform" or on a local PC which is allowed to connect to the Email Server IP Address on SMTP port 25.