I've discovered in syslog that postfix is reporting
postfix/cleanup: warning: mail_queue_enter: create file incoming/407669.31051: No space left on device
I also have problems with boinc suddenly stopping, apparently because it loses its connection to localhost. I don't know if this is connected.
df reports that all disks have plenty of space (root is only 40% used), no swap is being used, the waste bin is empty, and disks appear to be healthy. Any suggestions as to what might be causing this?
I'm using 14.10, with 3.16.0-25-generic.