Adding a Custom RBL
The Custom RBL feature lets you add custom RBLs and URIBLs to your account, that we do not normally monitor.
You can add a Custom RBL via the Manage ➡️ Custom RBLs section of the Portal:

Select the RBL type (either RBL or URIBL), and specify the Hostname of the RBL to add.
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This RBL must be set up like a "standard" DNSBL, as per this this description.
Once you've added the Custom RBL, you must remember to enable this RBL on your Monitoring Profile(s).