I am guessing but i assume the following help text should be $DEFAULT_RECIPIENTS not $DEFAULT_RECIPIENT_LIST.
      https://github.com/jenkinsci/email-ext-plugin/blob/master/src/main/webapp/help/globalConfig/defaultRecipients.html

          [JENKINS-11680] Default Recipients: misleading help text

            slide_o_mix Alex Earl
            ntfreak Spencer Oliver
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