[exim] failed to lock mailbox

Hi Guys,
I am seeing this error for certain users repeatedly:

defer (-9): failed to lock mailbox /path/to/john/doe/domain/name/inbox
(lock file)

I have search the internet for information on this and haven't had any
luck. It is causing several of our users alot of greif since it delays
email up to 24 hours! Can anybody help me out here? I am stumped.


--
John Hixson
Database Analyst/Programmer
InReach Internet
jhixson74@???
Phone: (209) 320 - 4377
Cell: (209) 403 - 3542