Often by default, postfix is configured to deliver email messages in mbox format. To convert mbox to Maildir
1. Edit /etc/postfix/main.cf and look for a line
# "Maildir/" for qmail-style delivery (the / is required).
home_mailbox = Maildir/
2. if procmail is used by postfix to deliver locally change /etc/procmailrc add a line
DEFAULT=$HOME/Maildir/
3. restart postfix
4. for new users to have Maildir/ after creation do
mkdir -p /etc/skel/Maildir/new
mkdir -p /etc/skel/Maildir/cur
mkdir -p /etc/skel/Maildir/tmp
5. convert all existing users to Maildir format using an effect perl script from qmail.org download it at http://www.qmail.org/convert-and-create
wget -c http://www.qmail.org/convert-and-create
then run it
sh convert-and-create
Note: for dovecot no need to change anything, it will autodetect mail box formats, very cool.