[Dovecot] migrating from mbox to maildir

[Erick Perez]

Hi all,
I have a locking problem in a Centos 4.4 linux machine with the
following config:
dovecot-0.99.11-8.EL4
postfix-2.2.10-1.1.el4

I use virtual mailboxes with postfix

currently I have dovecot with mbox as this:
default_mail_env = mbox:/var/mail/virtualmail/%d/%n

The problem is that mail is being delivered to mailboxes very very
slowly (sometimes up to 4 hours). In my logs, I have tons of these
messages:

mail postfix/virtual[24965]: CFB8878D04F: to=<REMOVED at REMOVED>,
relay=virtual, delay=51, status=deferred (mailbox
/var/spool/mail/virtualmail/REMOVEDDOMAINNAME/npitti: unable to lock
for exclusive access: Resource temporarily unavailable)

then someone told me that I need to change dovecot to maildir format.
Is there a reference to change format?
Or some comments about this?

Thanks in advance.