[exim] Recipient Callout Not Working

Hello all,

I am running exim as configured by cPanel 11 on my main email host. For
some reason, I cannot get recipient callout to work on this host. I need
to implement recipient verification because I have another exim system
running vexim that is acting as a relay for my cPanel system. Right now,
a bounce back is created by the relay system every time that the cPanel
system rejects a spam to the relay system creating collateral spam. I
think that I can mostly fix this simply by having the relay system
verify recipient callouts on the cpanel system, however, I cannot get
recipient callouts to work. I have added the following line to the
exim.conf file for the acl_smtp_rcpt ACL:

require verify = recipient/callout=defer_ok

However, a simple telnet session shows that the cpanel system is still
not doing the recipient callout since it continues to accept mail for
users that don't exist.

I have recipient callouts working correctly on other exim systems (which
are not running cpanel), so, I don't understand why this isn't working.

Any help on debugging this would be greatly appreciated.

BTW, exim -bV on the cPanel system reveals:

Exim version 4.68 #1 built 02-Sep-2007 10:20:27
Copyright (c) University of Cambridge 2006
Berkeley DB: Sleepycat Software: Berkeley DB 4.2.52: (February 22, 2005)
Support for: crypteq iconv() PAM Perl OpenSSL Content_Scanning
Old_Demime Experimental_DomainKeys
Lookups: lsearch wildlsearch nwildlsearch iplsearch dbm dbmnz
Authenticators: cram_md5 plaintext spa
Routers: accept dnslookup ipliteral manualroute queryprogram redirect
Transports: appendfile/maildir autoreply pipe smtp
Size of off_t: 8
Configuration file is /etc/exim.conf

Many thanks in advance!

Gordon Dickens