postinst is in fastforward 1:0.51-3.2.
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  | #!/bin/sh
set -e
test -e /var/lib/qmail/alias/.qmail-default ||
  echo '| fastforward -d /etc/aliases.cdb' > \
    /var/lib/qmail/alias/.qmail-default
 |