An RBL (real-time blackhole list) is a type of DNSBL (DNS black list) where DNS queries are used to determine whether or not to accept e-mail from a specific IP address. The goal is to reject spam at the MTA (mail transit authority) level.
RBLs through WestHost shared servers are managed by the admin team -- but your account is being protected by RBLs.
The best place to reject spam is at the MTA level. This allows us to minimize server resources (e.g. no mail delivery, no scanning by SpamAssassin, etc). The RBL lists available are both many and varied, each of which focuses on specific goals in the IP addresses they contain.
If you are unable to receive an important e-mail because the sender is listed in an RBL, the quickest fix is to have the sender use a different mail server. The long-term solution is to instruct the client to have the sender fix the problem causing them to be listed and request to have their IP address removed from the RBL.
How does WestHost request to have IP addresses removed from an RBL?
The procedure varies from list to list. Some will automatically prune out old IP addresses, while others allow you to fill out an online form. Most RBLs maintain a website with information about their list that can tell you more.