Chapter G. Diversions overriding a package's version of a file (from old Packaging Manual)
if [ remove = "$1" ]; then
dpkg divert package smailwrapper remove rename \
divert /usr/sbin/smail.real /usr/sbin/smail
Do not attempt to divert a file which is vitally important for the system's operation when
there is a time, after it has been diverted but before
the new version, when the file does not exist.