[Exim] Retry Configuration

Hello,

I have noticed that some mails on our Cpanel/WHM server running Exim, have
very late deliveries (1d to few weeks). On checking exim_mainlog, I found
that several entries of the form:

2004-04-08 12:00:38 1BBT36-0006Yd-LC == deepak@??? R=virtual_user
T=virtual_userdelivery defer (-9): failed to lock mailbox
/home/foo/mail/foo.com/deepak/inbox (lock file)
2004-04-08 23:37:48 1BBT36-0006Yd-LC == deepak@??? R=virtual_user
T=virtual_userdelivery defer (-52): Retry time not yet reached

What is disturbing is that after the first time that Exim tried delivering,
it found the inbox busy, probably due to an ongoing mail download. However,
the next retry attempt was directly after 12 hours, and even then it was
not due for a retry as per exim queue program.

I researched and found that the retry behaviour can be altered in exim.conf
by specifying the particular error and its retry sequence. As far as local
delivery errors are concerned, I only found two related to quota, but none
for the lock file problem. Has anybody faced such a problem, and got a
solution for it?

Btw, the example mail in question is yet in queue awaiting its turn even 36
hours after the 1st delivery attempt! :-(

Any help in this regard will be appreciated.

Regards,
Ninad