What is a blacklist
A blacklist is a mechanism used by system administrators to restrict spam from reaching their servers.DNSBL or RBL lists work by registering the IP addresses or networks of spammers in publicly accessible directories that mail servers can query from the Internet through a simple DNS query.
Administrators can configure their MTA agent or service responsible for receiving and distributing email messages to recipients to use one or more of these lists.
In this way, when the server receives an email, it performs a validation process that allows it to mark as spam or reject emails whose source IP address is listed or filtered as a spam generator.
Upon rejection, the sender may receive a sending error or a message indicating the reason for rejection. The messages contain terms such as listed, blackhole, denied, ...
If your Ip address is filtered in a public anti-spam list, you may have problems sending to some providers.
If you find yourself in this case, you can consult your provider's technical support to check the reason for the blacklisting.
How DNSBL lists work
A DNSBL list is a DNS zone whose A entries correspond to listed networks or IP addresses.
The A entries are registered with the octets in reverse order, which makes it possible to create a namespace for the subnets.
The reverse octet of IP address 188.8.131.52 is 184.108.40.206.
- It is possible to improve efficiency in the DNS server as it can group the content listed in different networks and subnets.
- It allows blocking entire networks or subnets with a single DNS record.
- Through the TTL of the DNS entry it is possible to control the listing time.
If the DNS server provides us with a DNS resolution, the standard indicates that our IP address is filtered.
The usual response codes refer to the loopback interface or 127.0.0.0.0/8. Within the loopback network, the responses may vary depending on the provider, allowing in some cases to obtain more information about the reason for the blocking. Blacklist providers usually have a usage guide on their web sites.
The convection indicates that the IP address 127.0.0.2 is used for testing and always appears as listed.Main blacklists
System administrators often set up one or more of these blacklists on their servers. To ensure that you receive your e-mails, it is necessary to check that your server's IP address is not filtered in the most important ones. The most popular among system administrators are Sorbs, Spamhaus and SpamCannibal.
The main difference between one and the other lists is the way they are fed. Inclusion can be based on spamtraps, reputation systems or systems that detect open Mail Relay systems and systems that filter bounces or bounces of mail called backscatter filters.
De-listing can be done automatically, when the reputation improves, or it may be necessary to make a request through a web form.