Whitelist IP netblocks for RBL checks while Greylisting is disabled

As a web-hosting provider, we would like to whitelist IP netblocks of common and trusted providers, so they can bypass RBL checks only.

This feature is already available with the Exim option "Whitelist: IP addresses that should not be checked against RBLs", but it is not feasible for whitelisting hundreds or even thousands of IP netblocks. It's also difficult to maintain across multiple of servers.

A much better solution is available with the Exim options "Exempt servers in the Greylisting “Common Mail Providers / Trusted Hosts” list from RBL checks", but it is part of the Greylisting feature, only while Greylisting is enabled. These options are tied to the Greylisting feature, even if the actual functionality is not directly related to it.

There's also the Exim Access List named "Only-verify-recipient", but this excludes IPs from all spam checks at SMTP connection time. We only want to exclude them from RBL checks and no other checks.

Basically, the exact same functions on the Greylisting page in WHM under the tabs "Trusted Hosts" and "Common Mail Providers" with the related Exim options would be required, but independently of the Greylisting feature. Since this feature already exists with Greylisting enabled, I believe it should be quite easy to implement/adapt for those who prefer not to use Greylisting.