If you notice the Exim graphs on your server have stop updating, you might want to check /var/log/munin/munin-node.log and see if you have lots of these entries:
tail /var/log/munin/munin-node.log ... Plugin "exim_mailstats" exited with status 768. ---- ...
You just need to remove the state file, and the graph will start updating again. Don’t worry about deleting the file, no data should be lost.