It's not so much about the mailbox permissions. All other mailboxes are in a directory that's writable by 'mail' (i.e. /var/mail). Your root mailbox is symlinked to a mailbox in the directory /root, which is not writable by 'mail'. That's why I suggest you create a subdirectory named 'mail' under /root that is writable by 'mail'. Still not sure about the symlink being acceptable for Sendmail, but one step at a time.
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