RBL servers are free public services that keep track of blacklisted IP addresses. Getting your public IP address listed in an RBL server guarantees emails from your IP address will end up in the junk folder. This article discusses some important tips on ensuring your IP address stays out of these lists.
IP addresses in an RBL server fall into two categories, and it is essential to identify your IP before trying to mitigate the problem.
Such RBL servers include SpamHaus SBL, SpamHaus XBL, SORBS, and many others. The most common reason for getting your IP address listed is incorrect configuration on your email server or an improperly configured firewall.
The easiest way to ensure is to click Tools/Run Diagnostic Check - Outbound if you're using Xeams. This will run comprehensive tests against your server to ensure all the email best practices are met. If you're not using Xeams, refer to this page for instructions that work with any email server.
Monitoring traffic for outbound TCP port 25 on your firewall is essential. The easiest way is to block this traffic from every IP address but the IPs that run your email server. Moreover, using software like Xeams that can log every outbound communication and is capable of sending alerts if unusual activity is detected is important.
Such RBL servers include SORBS DUHL and SpamHaus PBL, which include IP addresses that are not supposed to send outbound emails without authentication, such as addresses designated as dynamic. Many administrators incorrectly assume they can send outbound emails if their IP addresses do not change often or if they use a dynamic DNS service, such as NoIP or Oracle's DYN. Since your IP address is not classified as static, you will always see it in such lists, and attempts to remove it will be futile.
Refer to this page about the challenges you must overcome to run an email server behind a dynamic IP.