exim dead but subsys locked

Hi, Every day im waking up to a couple of clients telling me they are not able to send and receive email. When I check the status of Exim, I see the error: "exim dead but subsys locked" A simple >service exam restart fixes this but I need to get to the root of why its happening. Ive googled and read docs that it usually one of two things 1) The presence of /etc/eximdisable file. I have checked and this file does NOT exist. 2) Out of disk space It seems fine: see attached file: 58867 The only thing I can think of is that backup is run overnight and this creates a local copy of files before ftp'ing them off server. This "might" be creating a low disk space issue but I don't know how to check. This is something thats just happened in the last week or two, but seems to be happening daily or every other day and is happening overnight at some point. I also don't actually know what im looking for in log files to be able to help Any help from here would be invaluable. Thanks UKD.