procmail error generated mail loop

Hi, all:
We had a user who misconfigured his .forward and procmailrc file, which 
generated undeliverable mail to loop and filled our /var/mail spool.  I 
wonder if there is a way to prevent this type of undetected loop.
This is what he has in his homedirectory:
.forward: "|/usr/local/bin/procmail"

.procmailrc:
     :0 c
     2  *
     3  !LOGIN(_at_)pacbell(_dot_)net

We are running sendmail 8.11.2
Here is what we have in sendmail.cf

 Mlocal,         P=/usr/local/bin/procmail, F=lsDFMAw5:/|@qSPfhn9, 
S=10/30, R=20/40,
  1453                  T=DNS/RFC822/X-Unix,
  1454                  A=procmail -Y -a $h -d $u

Thanks for any help you can give

Xiaoyan

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