[Exim] 'and' subcondition evaluation

In the following, I'm trying to make the execution of spam.csh
conditional on the existence of the file
/var/exim/userspamcontrol/$local_part/enablescan

The docs say the subconditions are evaluated left-to-right and
stop when the first one fails, but in this case spam.csh is getting
executed regardless...

  condition =   ${if and { \
                           {exists{/var/exim/userspamcontrol/$local_part/enablescan}} \
                           {eq \
                               {${run{/bin/csh /usr/exim/spam.csh $local_part ${spool_directory} ${message_id}}{true}{true}}} \
                               {true} \
                           } \
                         }{true}{true} \

What am I doing wrong?

Cheers,
Terry.