For a non-root user, postdrop fails with
"warning: mail_queue_enter: create file maildrop/###.###: Permission denied"
where, ###.### is some number. The reason is that postfix isn't installed with
setgid (install.cf), so postdrop runs with the permissions of the user who runs
it (postdrop is called by postfix's sendmail replacement, which is called by
mail programs like mutt). But /var/spool/postfix/maildrop is installed with
user/group postfix/postdrop and isn't world-writable. Postdrop needs to write
to that directory, so if setgid isn't set in the postfix install/build, which
appears to not be, /var/spool/postfix/maildrop should be world-writable (the
Mandrake postfix install does this). However, the better answer is to set
setgid on the postfix install.
The URL above has more info: Scroll down to "Protecting your maildrop
latest postfix fixes this problem .. thanks woodchip :)