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I will talk with UnrealIRCD devs to change it to work as InspIRCD by sending 421 raw instead of server notice, then I am gonna test it to see how will works and reply you back with more details.

If LIST is not supported, sending a 421 is a better way to inform the client. A NOTICE is not because it could be in many different languages, and even in the same language it could be phrased differently by different server administrators. A 421 numeric is the better way to do this.

That said, mIRC does not currently support 421 for LIST. I can add it to the next version however.