prerm is in mailutils 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 6 7 8 9 10 | #!/bin/sh
set -e
       
if [ "$1" = "remove" ]; then
    for bin in dotlock frm from messages movemail readmsg; do
        update-alternatives --remove $bin /usr/bin/$bin.mailutils
    done
    update-alternatives --remove mailx /usr/bin/mail.mailutils
fi
 |