In some instances, even after an IP has been removed from an RBL the recipient mail server may still reject the message for up to 24 hours. 

While the RBLS are real time. This is what that (R) stands for. However, not every server on EVERY device uses it this way.

In the ideal use-case, an email service or spam filter will query the RLB directly. But there are instances where some firewalls, appliances or filters use an aggregate subscription to the RBLS which are not synced to their devices in real time. They typically do a DNS zone transfer from their vendors on a schedule. 

In these instances, the RBL list they are viewing is not a real time sync and they may still reject mail even after the validated removal of an IP from the RBL. We have seen as long as 24 hours after the IP has been removed for these appliances to get updated.

In some appliances, there is a manual button to check for updates. However, for some firewalls there is no way to force a resync. The recipient mail server administrator will have contact their vendor.