Realtime black lists are web site services that keep track of the domain name or ip addresses where possibly the spam emails originate from. The purpose behind the service is to allow IT Mail Administrators to query the database and block messages that are listed in there in order to block spam. There are scores of websites that offer this type of service and most of them provide the service at no cost.
If your IP address is listed in an RBL then the chances are that the emails that you send from your network to your clients can get blocked even though it may not be spam. Most anti-spam filters query RBLs and if your IP address is listed in one of the RBLs then these filters may not allow your e-mails to go through and block them as spam.
Several reasons this can happen. Viruses, spywares, Trojans can also be causing a workstation on your network to send out spam e-mails. If your company sends out bulk e-mails such as newsletters or mass e-mailings of some sort then chances are one of these e-mails may have been reported to the RBL by someone who thought it was a spam. Some of the RBLS services also work with ISPs and setup automatic probes to detect mass mailings and where it is originating from and this can also trigger a red flag that can have your IP black listed.
Visit the RBL website that has you blacklisted and find a link there that would allow you to request a removal from the black list. Use our RBL Search Tool to determine if your domain/IP is black listed. The result page provides link to the RBL website by clicking on the RBL name.