PREUN

/bin/sh

if [ $1 -eq 0 ]; then
  update-alternatives --remove mutt /usr/bin/mutt-normal
fi

POSTIN

/bin/sh

update-alternatives --install /usr/bin/mutt mutt /usr/bin/mutt-normal 10

Triggers

mutt < 1:1.5

/bin/sh

update-alternatives --install /usr/bin/mutt mutt /usr/bin/mutt-normal 10