Problems exporting CLIENT_ADDRESS to procmail

I'm running postfix 2.3.4 and procmail 3.22, with the following in
Postfix's main.cf:

    mailbox_command = /usr/bin/procmail -p

However, I am not seeing CLIENT_ADDRESS in procmail's environment. I've
also tried:

    mailbox_command = /usr/bin/procmail -p -a "$CLIENT_ADDRESS"

and

    export_environment = TZ MAIL_CONFIG LANG CLIENT_ADDRESS

with similar results. Why can't I get at the IP address from procmail?

-- 
Unabashedly littering the information superhighway with detritus like
this for over 15 years now.

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