Why can't I send mail to root?

[Robert Storey <y2kbug@ms25.hinet.net>]

Using the "mail command, I can send a message to another user, in this
case, user "zig":

  bob@sonic:~$ mail zig -s "crap"
  this is crap
  Cc: bob@sonic:~$ su zig
  Password: 
  zig@sonic:/home/bob$ mail
  Mail version 8.1.2 01/15/2001.  Type ? for help.
  "/var/mail/zig": 1 message 1 new
  >N  1 bob@localhost.loc  Sun May  4 13:59   13/428   crap
  & 1
  Message 1:
  From bob@localhost.localdomain  Sun May  4 13:59:19 2003
  To: zig@localhost.localdomain
  Subject: crap
  Date: Sun,  4 May 2003 13:59:19 +0800 (CST)
  From: bob@localhost.localdomain

  this is crap

  & 
  At EOF
  & q
  Saved 1 message in /home/zig/mbox


All well and good. But I cannot send mail to root, even though I get no
error message when I make the attempt:

  bob@sonic:~$ mail root -s "crap"
  this is crap
  Cc: bob@sonic:~$ su -
  Password: 
  root@sonic:~# mail
  No mail for root
  root@sonic:~# 

Any ideas on why this is?

regards,
Robert