Could not complete sender verify callout

2016-02-11 09:33:49 H=external.mailserver.com [86.75.30.9]:54245 X=TLSv1:DHE-RSA-AES256-SHA:256 CV=no F=<root@host.domain.com> temporarily rejected RCPT <user@domain.com>: Could not complete sender verify callout
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WHM >> Service Configuration >> Exim Configuration Manager:

Sender Verification Callouts: On –> Off