Root mailbox permissions being changed

Mailbox owned by root account itself => bad idea

root's "super powers" should not be required to read emails to root.
Creating needless risk should be avoided.

You SHOULD create alias redirecting email addressed to too to non privileged account.

/etc/mail/aliases

root: name-of-non-privileged-account

WARNING: sendmail requires running command newaliases after modifying aliases file.