I needed to replace my mail file with a cleaned version (all mail does on this machine is monitor cron jobs--so there's no reason not to delete mail more than 'n' days old).
The file has read/write permissions for my user account, with no rights to any other user or group. It's group ownership is mail group because the setguid bit is set on /var/spool/mail.
I was not a member of the mail group.
drwxrwsr-x 2 root mail 4096 Sep 2 15:17 . drwxr-xr-x 14 root root 4096 May 10 09:27 .. -rw------- 1 myuser mail 219722 Sep 2 04:56 myuser
I couldn't move or delete this file until I made myself a member of the mail group.
My question is 'Why?'. Why would making myself, who shows to have permission to both read and write this file, a member of a group which has neither give me permission to move and delete it?