[Exim] Error message

I have been running version 3.02 since November without problems.

I have noticed, however, that I occasionally get message that are hung and
when I try to cancel them using Eximon the message log is empty and the
"Give up on message" command gives me an error like:

/usr/exim/bin/exim -v -Mg 12YBbO-00029m-00
delivering message 12YBbO-00029m-00
LOG: 5 MAIN
Spool file is locked

I do plan on upgrading this summer but what is going on.

Thanks,

-- Jack Elliston