postinst is in mailutils-comsatd 1:2.99.99-1ubuntu2.
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 | #!/bin/sh
set -e
update-inetd --group MAIL --add "biff \tdgram \tudp \twait \troot \t/usr/sbin/comsatd comsatd -i"
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