"Mailbox is read-only"

[Edmund GRIMLEY EVANS]

When I run mutt I find myself looking at /var/spool/mail/edmund. I
can't delete anything because "Mailbox is read-only". Can someone tell
me what I should do? Should I arrange for mail to be transferred from
/var/spool/mail/edmund to somewhere else? How? Does mutt do this, or
some other program?

$ ls -ld /var/spool/mail /var/spool/mail/edmund 
drwxrwxr-x   2 root     mail         1024 Aug  2 14:15 /var/spool/mail
-rw-rw----   1 edmund   mail       415001 Aug  2 14:15 /var/spool/mail/edmund