[Dovecot] Intermittent "cannot update mailbox" problems w/ postfix & dovecot

[John Reddy]

I hate asking questions on mailing lists, but I've exhausted my
googling and FAQ trawling skills.  If I've missed the pertinent doc,
please tell me where to RTFM.

Environment:
I'm running postfix 2.3.2 on a Solaris 10 server.  Mail goes into
standard mbox format in /var/mail.  Users can access their mail via
shell (pine, mutt), POP3 or IMAP, and the pop3/imap server is
dovecot-1.0.rc15.

Problem:
For a few users, mail occasionally fails to deliver.  The mail is
accepted by smtpd, passes cleanup and qmgr without incident.  When it
gets to local delivery, though, local complains that "cannot open
file: Permission denied".  A sample log entry (slightly sanitized):

Dec  7 09:42:05 solaris-host local[18332]: [ID 197553 mail.info]
D35C3445C: to=<bubba at solaris-host.sample.com>, relay=local,
delay=0.17, delays=0.09/0/0/0.08, dsn=5.2.0, status=bounced (cannot
update mailbox /var/mail/bubba for user bubba. cannot open file:
Permission denied)
Dec  7 10:52:26 solaris-host local[20775]: [ID 197553 mail.info]
670C9445D: to=<jack at solaris-host.sample.com>, relay=local, delay=15,
delays=8.9/0/0/6.5, dsn=5.2.0, status=bounced (cannot update mailbox
/var/mail/jack for user jack. cannot open file: Permission denied)

Does anyone have any advice on how I might get better details about
why postfix is having trouble accessing the file?  Has anyone run into
this before?  My initial suspicion is a lock file from the users' POP3
connections...

Thanks in advance for any help.

-John