With dovecots zlib plugin, it's possible to have compressed mails in the users maildir.

I've written a script to compress all mails on my mailserver:

#!/bin/sh

compress_maildir () {
cd $1
DIRS=`find -maxdepth 2 -type d -name cur`
for dir in $DIRS; do
　　echo $dir
　　cd $dir
　　FILES=`find -type f -name „*,S=*“ -not -regex „.*:2,.*Z.*“`
　　#compress all files
　　for FILE in $FILES; do
　　NEWFILE=../tmp/${FILE}
　　#echo bzip $FILE $NEWFILE
　　if ! bzip2 -9 $FILE -c > $NEWFILE; then
　　echo compressing failed
　　exit -1;
　　fi
　　#reset mtime
　　if ! touch -r $FILE $NEWFILE; then
　　echo setting time failed
　　exit -1
　　fi
　　done
　　echo Locking $dir/..
　　if PID=`/usr/libexec/dovecot/maildirlock .. 120`; then
　　#locking successfull, moving compressed files
　　for FILE in $FILES; do
　　NEWFILE=../tmp/${FILE}
　　if [ -s $FILE ] && [ -s $NEWFILE ]; then
　　echo mv $FILE $NEWFILE
　　mv $FILE /tmp
　　mv $NEWFILE ${FILE}Z
　　else
　　echo mv failed
　　exit -1
　　fi
　　done
　　kill $PID
　　else
　　echo lock failed
　　exit -1
　　fi
　　cd - >/dev/null
done
}

compress_maildir /home/user/.maildir/
chown -R username:users /home/user/.maildir/
If using this script, please do it after an backup/copy of the maildir, the script is far away from being perfect

If the Script is rerun, it checks the files for a Z at the end of the filename, so the file isn't recompressed.

With compressed mails i saved ~25% of space.