Cannot see mail in Inbox in Outlet Express 5 client

Sendmail server:  Redhat Linux 7.2
Sendmail user ID:  a)  johnc     b)  peterc
Mail client for users johnc (myself) and peterc:  MS Outlook Express 5

1)  I send a mail to peterc.  Then I click Send/Recv button.  Outlook seems 
to send the mail correctly because there's no error message and the CPU 
seems to process something for about 2 seconds.
2)  However, peterc logs to his Outlook Express client and click Send/Recv 
mail.  Some CPU processing seems to go on for 1-2 seconds.  But there's no 
mail in Inbox.
3)  If peterc sends mail to me (johnc), I can see the mail either in my 
Inbox.

Troubleshoot:
4)  Then I go to the Sendmail Linux server and log in as root.  As a root 
user, I send mail to johnc and peterc.  Now peterc and johnc could see one 
mail from root user in their respective Inbox.

Why does this happen?  Pls help.