JH/12 Cutthrough: disable when verify option success_on_redirect is used, and
when routing results in more than one destination address.
+JH/13 Cutthrough: expand transport dkim_domain option when testing for dkim
+ signing (which inhibits the cutthrough capability). Previously only
+ the presence of an option was tested; now an expansion evaluating as
+ empty is permissible (obviously it should depend only on data available
+ when the cutthrough connection is made).
+
Exim version 4.87
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