Procmail as local mailer problems

 This is on a SunOS4.1.3 machine with sendmail 8.6.8.1.

 I need to make a backup of email sent to a certain user and thought
 procmail can do it. So I configured procmail as local mailer in sendmail.cf

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

 (taken from cf/mailer/procmail.m4 from sendmail-8.8.7) and created
 /etc/procmailrc

:0c
* ^TO_user
/path/to/backup

 Now, with this setup mail to user is stored in the backup file,
 but mail to all other users fails to deliver. I figured that
 I need to set DEFAULT, but how to? Mail for user goes
 to /var/spool/mail/user. How do I pass the "user" part to procmail?

 Can I just say

DEFAULT=/var/spool/mail/$2

 in /etc/procmailrc? I couldn't find anything useful in the man page.

 Thanks for any insights.