preinst is in msmtp-gnome 1.4.26-1.
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 | #!/bin/sh
set -e
# Divert msmtp to msmtp.orig so the binary from msmtp-gnome can replace it.
if [ "$1" = install ] || [ "$1" = upgrade ] ; then
dpkg-divert --add --package msmtp-gnome --rename \
--divert /usr/bin/msmtp.orig /usr/bin/msmtp
fi
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