An RBL is a third-party listing of IP addresses which have been reported or detected to have recently sent spam. We do not maintain or have any control over listings/removals with those third-party providers; we simply check against their listings. Some common lists are maintained by SpamHaus, SpamCop, CBL, and Barracuda.
As ‘real-time’ would suggest, these listings can occur shortly after a spam outbreak has been detected, and usually require that the administrators for that sending IP take action to address the spam issue (which may be due to a compromised email password; malicious mailing scripts on the server; etc.) and submit a delisting request, acknowledging that they have corrected the issue. Most RBLs process delisting requests within a few hours, some less than 30 minutes.
Checking a sender’s IP address against reputable RBLs is a common spam-prevention measure on mailservers which helps to reduce a large portion of the spam that would otherwise be delivered to inboxes, as those listed IPs are confirmed sources of active spam.
RBL checks are enabled by default on all accounts on our servers. If a sender is receiving a bounceback message stating that their email was rejected due to their IP being listed in an RBL, please request that they provide that message to their email administrators so that they can investigate and correct that issue for them.
These checks can be disabled per receiving domain (i.e. the domain you have your email hosted with us, not the sender’s domain) by the account holder’s request, however this will very likely lead to an increase in spam as mail being sent to your domain will not be filtered against a list of known spamming IP addresses. If this is an option you would like to proceed with, please contact support to request that check be disabled. For account verification, please include the support pin from the top left corner of your client area in your request.