Using 'Immediate access Residential' zone, what error will I see if I target domain which is not in the list?

When targeting domains that are not yet part of the Immediate access residential the request will result in an error
You can identify the domain is not approved yet by collecting the x-luminati-error header:
"Requested site is not available for immediate residential access mode.
Ask your account manager to get full residential access for targeting this site.