# If currently using bsd-mailx:
apt remove bsd-mailx
# - bsd-mailx does not support maildir
apt install mailutils
# - which does support maildir
apt install mb2md
# As each user:
mb2md -m
# - converts delivery location from /var/mail/${user}
#   to ~/Maildir/new, creating ~/Maildir structure
cd; mb2md -s mbox -R
# - Populates Maildir from contents of ~/mbox

# Edit login, sshd and su in /etc/pam.d
# - add to the pam_mail line dir=~/Maildir

# It is recommended to edit /etc/login.defs
# to change MAIL_FILE to Maildir/