HS,
Your thread gave me the idea (that and I was having Evolution SMTP send mail errors, along with Postfix errors). My problem was where you said, "First, you need a mailbox, preferably with all of root's old mail already in it." Unfortunately root didn't have any mail nor a mailbox that I could work with.
If I try to send a mail to root@localhost, Postfix will send me a MAIL-DAEMON "Undelivered Mail Returned to Sender" to my real email account. If I send an email to root@localhost.localdomain it goes into the ether, lost forever (because I am not running Postfix).
I guess if I enabled the root account (gave it its own password, etc.) it may create a root email postoffice box, but it's nothing I wish to try at the moment as I am already used to suid.