postinst is in dovecot-sieve 1:2.2.22-1ubuntu2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16  | #!/bin/sh
set -e
if [ "$1" = "configure" ]; then
  CONFFILES="conf.d/90-sieve.conf \
  conf.d/90-sieve-extprograms.conf"
  for conffile in $CONFFILES ; do
    # Tell ucf that the file in /usr/share/dovecot is the latest
    # maintainer version, and let it handle how to manage the real
    # configuration file in /etc/dovecot.
    ucf --three-way /usr/share/dovecot/$conffile /etc/dovecot/$conffile
    ucfr dovecot-sieve /etc/dovecot/$conffile
  done
fi
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