Implements the cron script described on http://wiki2.dovecot.org/Plugins/Zlib for compressing old maildir messages. Use only after enabling the Dovecot zlib plugin.

dovecot-zlib-cron-compressor.sh

#!/bin/bash
# This is implementation of the script that compresses old maildir messages
# as described in http://wiki2.dovecot.org/Plugins/Zlib
#
# Copyright (C) 2016 by Konstantin Ryabitsev
#
# MIT License
#
# Permission is hereby granted, free of charge, to any person obtaining a copy of
# this software and associated documentation files (the "Software"), to deal in
# the Software without restriction, including without limitation the rights to
# use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
# of the Software, and to permit persons to whom the Software is furnished to do
# so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
#
# Configurable parameters
# Toplevel with all the mailboxes
MAILTOP=/var/vmail

# What should we use to compress the messages
# NOTE: check that your zlib plugin was actually compiled to support that.
# E.g. on RHEL7 there is no support for xz, only gz and bzip2
COMPRESSOR=/bin/bzip2

# Level of compression to use (6 is the default).
LEVEL=9

# Compress messages older than this many days. You probably don't want to
# make this too small, as we want to compress mail that's not likely to be
# frequently touched by clients.
OLDERTHANDAYS=90

# Where do we find maildirlock (part of dovecot)
MAILDIRLOCK=/usr/libexec/dovecot/maildirlock

# How long should we wait for the lock before moving on (seconds)
LOCKTIMEOUT=5

# List any directories under MAILTOP that you want to exclude
EXCLUDEUSERS="archive archived-and-removed"

# This should have "cur" if you only want to compress non-new mail, or "cur new"
# for everything.
INCLUDEFOLDERS="cur new"

# End configurable parameters

# Script begins here
TMPFILELIST=`mktemp --suffix='MAILCOMPRESSOR'`

# First, find all toplevel mailboxes
find $MAILTOP -maxdepth 1 -mindepth 1 -type d -print0 | while read -d $'\0' USERDIR; do
  EXCLUDED=0
  for EXCLUDEUSER in $EXCLUDEUSERS; do
    if [ "${USERDIR}" == "${MAILTOP}/${EXCLUDEUSER}" ]; then
      EXCLUDED=1
      break
    fi
  done
  if [ "${EXCLUDED}" == "1" ]; then
    continue
  fi

  # Now find all dirs we want
  for INCLUDEFOLDER in $INCLUDEFOLDERS; do

    find "${USERDIR}" -type d -a -name "${INCLUDEFOLDER}" -print0 2>/dev/null | while read -d $'\0' FOLDER; do
      MAILFOLDER=`dirname "${FOLDER}"`
      TMPFOLDER="${MAILFOLDER}/tmp"
      # If tmpdir does not exist, something is odd
      if [ ! -d "${TMPFOLDER}" ]; then
        continue
      fi

      # Now find any old files that haven't been compressed yet
      # We store it in a temporary file so we avoid locking a maildir if the list is empty
      find "${FOLDER}" -iname '*,S=*[^Z]' -mtime +${OLDERTHANDAYS} -print0 > $TMPFILELIST
      if [ -s "${TMPFILELIST}" ]; then
        # Compress first
        cat $TMPFILELIST | while read -d $'\0' EMAILFILE; do
          # Is it still there?
          if [ -f "${EMAILFILE}" ]; then
            # Get the tmp location for this file
            BASEFILE=`basename "${EMAILFILE}"`
            TMPFILE="${TMPFOLDER}/${BASEFILE}"
            # Has it been compressed already?
            rm -f "${TMPFILE}"
            if ! /bin/file $EMAILFILE | grep -q 'compressed data'; then
              # Compress it into TMPFILE
              ${COMPRESSOR} -${LEVEL} -c "${EMAILFILE}" > "${TMPFILE}"
            else
              # What happened? Why does it not end with Z? Bad previous run?
              # Just copy it over then
              cp "${EMAILFILE}" "${TMPFILE}"
            fi
            # Set the same mtime
            touch -r "${EMAILFILE}" "${TMPFILE}"
            # Set the same modes
            chmod --reference "${EMAILFILE}" "${TMPFILE}"
          fi
        done
        # Now we lock maildir and move all these files to replace old ones
        if LOCKPID=`${MAILDIRLOCK} "${MAILFOLDER}" "${LOCKTIMEOUT}"`; then
          # Iterate through the list again
          cat $TMPFILELIST | while read -d $'\0' EMAILFILE; do
            BASEFILE=`basename "${EMAILFILE}"`
            TMPFILE="${TMPFOLDER}/${BASEFILE}"
            if [ -f "${TMPFILE}" -a -f "${EMAILFILE}" ]; then
              mv -f "${TMPFILE}" "${EMAILFILE}"
              mv -f "${EMAILFILE}" "${EMAILFILE}Z"
            fi
          done

          # Free up the maildir
          kill $LOCKPID
        else
          # Could not lock maildir, so I guess just clean up
          cat $TMPFILELIST | while read -d $'\0' EMAILFILE; do
            BASEFILE=`basename "${EMAILFILE}"`
            TMPFILE="${TMPFOLDER}/${BASEFILE}"
            rm -f "${TMPFILE}"
          done
        fi
      fi
    done
  done
done

rm -f $TMPFILELIST

It works, but all compressed mails are owned by root and I have to change ownership to mail user.
Also