[exim] Spool file is locked?

I frequently (a few times a day) see the following message in my mainlog
(with different message ID's, of course):

2005-01-18 18:40:17 1Cr2xF-00045e-2C Spool file is locked (another
process is handling this message)

What does this mean? Is it a normal occurance when a queue-run tries to
deliver a message while it is being accepted over SMTP? Or is it a sign
of some kind of error in my configuration?

Thanks,
- Marc