[Exim] Semantics of freeze_exec_fail

In a pipe transport, does freeze_exec_fail cause the message to be frozen
only in the case where the fork/exec fails, or will it also freeze the
message if the pipe command returns an error ?

If only the former is true, is there any way to cause the message to be
frozen if the pipe command returns non-zero ?

Thanks,
James.
-- 
 "Yield to temptation --             | Consultancy: james@??? 
  it may not pass your way again"    | http://www.cloud9.co.uk/james
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        - Lazarus Long               |              James Fidell