A Registered Block List (RBL) is a public service that collects and catalogues information relating the ‘reputation’ of a DOMAINS and IP addresses, commonly refered to sending email server reputation (whis is not actually true). The reputation is an overall ranking of how reliable that server (IP and domain) is for legitimate email, and in many cases how unreliable a server might be.

When you operate a mail server you are dealing not only with inbound mail for your users, but also outbound mail that your users are sending. Due to the ever increasing technology demands for businesses to conduct their communications online there are always going to be those that choose convenience over security. This might happen when a user decides to choose a weak password over a strong one, to ease their day-to-day workflow however this has the detrimental effect of opening a vulnerability on the system.

When that vulnerability is exploited in most cases it is done to generate spam emails, that send from the server to other unsuspecting users on the internet. When this happens the server’s reputation is logged as decreased on the RBL and if it continues to occur then the server’s sending address is placed on a blacklist.

The RBL itself is a service much like a phone book, and the blacklist is a mark against an IP address AND DOMAIN, which indicates that emails originating from that address have been proven as malicious and shouldn’t be trusted. This will likely manifest in emails sent to other servers bouncing back if those recipient mail servers are checking the same RBL to validate sending servers.

Usually, a shared hosting network is protected by outbound spam filtering, which has proven to reduce outbound spam originating from our network by 99%.  While the vast majority of email sent from our network is legitimate, some clients may still send email that is flagged as spam by a mail filter or recipient outside of our network, resulting in the sending IP as being blacklisted.

An IP is deemed blacklisted if it is found on an RBL listed at MX Toolbox which is an external service we monitor 24/7 for updates. If an IP we manage is listed as blacklisted, we will endeavor to remove the blacklist or change the mail delivery IP address.

If an email is being blocked due to the actions of the recipient network, then the recipient will need to contact their provider to have the issue resolved.  Email providers blocking IP addresses and domains, do so at their own discretion and generally only action unblock requests from their own customers, not requests of a third party.  If this occurs, better to contact the recipient by other means and send them a copy of the bounce message so they can forward it to their provider.

Some providers will block and entire IP range or even country in order to reduce spam.  Not only is this highly impractical, it’s also a very rudimentary way to solve spam problems.  There are many other best practice methods to reduce spam, rather than just simply blocking IP addresses.