df show's
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/main-root
                      75732080  71884948       172 100% /
/dev/md1                101018     19240     76562  21% /boot
none                    256888         0    256888   0% /dev/shm
[root@mumbles ~]#
I guess it thinks the hard drive us still full, although we have deleted at least 8,000 emails and 2Gb's worth of data.
I'm awaiting the correct message from those on site regarding what Thunderbird reports.