[exim] MBX format and UW-IMAP

Hello,

I have been experimenting with implementing shared mailboxes using
MBX format, but I seem to have run into a tricky problem with file
locking and I was wondering if anyone else has seen this and can
advise on a fix.

Briefly, once the shared mailbox has been accessed via UW-IMAP (2004g),
the next time that Exim (4.60) delivers to that mailbox it leaves
behind a lockfile in /tmp. Whilst this does not prevent subsequent
deliveries to the mailbox, it seems to prevent the IMAP client from
seeing them. If I stop and re-start the IMAP client it then complains
that it cannot lock the mailbox, and so can no longer access it at all.

Having removed the lockfile from /tmp I experimented with using the
exim_lock command, which worked fine on the mailbox until I had viewed
it via IMAP, after which, when exiting from the shell launched by
exim_lock, I got the output:

exim_lock: fcntl() failed: Resource temporarily unavailable
exim_lock: /tmp/xxxxxxxx not unlinked - unable to get exclusive mailbox lock
exim_lock: /tmp/xxxxxxxx closed
exim_lock: mailbox closed

If I have read the documentation correctly, this implies that the
IMAP server is holding a lock on the mailbox file. I don't know whether
that lock is exclusive or shared, but perhaps that's irrelevant because
if Exim can't get an exclusive lock on it then it won't remove the
lockfile from /tmp, and I presume it is the presence of that which then
prevents IMAP from accessing the mailbox.

Am I overlooking something here or completely misunderstanding the way
that the locking is supposed to work whilst still supporting simultaneous
access to the mailbox?

For the record, this is running under Solaris 9.

Thanks,

Dave