[Dovecot] cannot update mailbox - unable to lock for exclusive access

[1st WebDesigns]

Hi there,

We're using Dovecot version 1.0.7 and Postfix, and since upgrading our 
Linux box we're getting this in the maillog:

Nov  8 07:49:11 server1 postfix/local[27901]: 04B8E7081DA: 
to=<xyz at xyz.com>, orig_to=<abc at abc.com>, relay=local, delay=19, 
delays=0.07/0/0/19, dsn=4.2.0, status=deferred (cannot update mailbox 
/var/spool/mail/abc for user abc. unable to lock for exclusive access: 
Resource temporarily unavailable)

Postfix is currently set to:
mailbox_delivery_lock = fcntl, dotlock

Dovecot has this:
mbox_read_locks = fcntl
mbox_write_locks = fcntl dotlock

I've scoured the web and tried all kinds of different locking mechanisms 
and combinations to no avail.  The mail is eventually delivered but on a 
busy day this can take several hours.

In the evening it generally doesn't happen, which leads me to believe it 
occurs during the day when people have their mail clients open. 
However, this particular issue doesn't affect every user that has their 
mail client open, only some!

We currently have the mailboxes in mbox format - yes I know we should 
upgrade to maildir which we will eventually, however we've been using 
mbox for the last 10 years and this has only become an issue since 
upgrading.

What I have discovered this morning is a Dovecot connection that is open 
for 6 minutes before disconnection.  During this 6 minutes the above 
problem occurs (new mail cannot get an exclusive lock).  The same user 
will have connected and disconnected several times before and after, 
with a connection taking no more than a few seconds.  But it seems 
sometimes the connection is taking longer than it should and I suspect 
the problem lies here.

Please can anyone help!

Thank you,

G