unset was to force an expansion failure. That still works, and
an empty string is now equivalent.
+PP/19 Renamed DNSSEC-enabling option to "dns_dnssec_ok", to make it
+ clearer that Exim is using the DO (DNSSEC OK) EDNS0 resolver flag,
+ not performing validation itself.
+
Exim version 4.80.1
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