No space left on device

[R Kimber]

I've discovered in syslog that postfix is reporting

postfix/cleanup[31051]: 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.

- Richard
-- 
Richard Kimber