Description of Problem:

When a new user account is created by using useradd command,
a new empty mailbox is created in /var/spool/mail/newuser
with these permissions:
-rw-rw----   newuser  newgroup 
But if two (or more) users have the same GID, they can read
(and modify) each others mail. 

Version-Release number of selected component (if applicable):
shadow-utils-20000902-12

This is what I found in changelog of shadow-utils:
It probably has something to do with the problem mentioned
above.

* Wed May 23 2001 Bernhard Rosenkraenzer <bero> 20000902-1
  - Create an empty mailspool when creating a user so non-setuid/non-setgid
    MDAs (postfix+procmail) can deliver mail (#41811)

How Reproducible:

every time

Steps to Reproduce:
1. useradd -g users newuser1
2. useradd -g users newuser2
3. Now the newuser1 and newuser2 can read and modify each others mail

Actual Results:
see above

Expected Results:
Even if this is done deliberately, I would expect useradd command
to WARN about this potential security flaw.

Additional Information: