Masquerade problem? (sendmail/maildrop/tmda-1.1.9)

Hello, tmda newbie here.

I'm probably missing something obvious, but...

TMDA is installed on our IMAP mailstore and appears to work splendidly 
as long as the auto-generated address in confirmation emails is the 
machine-name. However, it doesn't work (ie - the reply to the 'confirm 
your address' email with the Magic Ticket results in another 
confirmation request) if I set the confirmation address to the 
masqueraded site name, using one of CONFIRM_ADDRESS or HOSTNAME in 
~/.tmda/config.

A poke through the archive revealed that hand-setting HOST in 
.mailfilter might also do the right thing, but I've had no luck with it.

Unfortunately, I'm by no means a Python expert, so staring at the code 
has availed me naught thus far.


tmda-keygen -V reports:
TMDA/1.1.9 "Jura" (Python/2.5.0 on HP-UX-B.11.23-9000-800-32bit)

.tmda/config is:
# -*- python -*-

# ~/.tmda/config
#
# TMDA configuration file.  See
# http://wiki.tmda.net/ConfigurationVariables for descriptions of all
# the available options.

########################################################
# Required settings.  One from each section is required.
########################################################

#MAIL_TRANSFER_AGENT = "qmail"
#MAIL_TRANSFER_AGENT = "postfix"
#MAIL_TRANSFER_AGENT = "exim"
MAIL_TRANSFER_AGENT = "sendmail"

#RECIPIENT_DELIMITER = "-"
RECIPIENT_DELIMITER = "+"

#DELIVERY = "~/Mailbox"
#DELIVERY = "/var/mail/%s" % os.environ.get("USER")
DELIVERY = "~/Maildir/"
#DELIVERY = "|/usr/bin/procmail ~/.procmailrc-tmda"
#DELIVERY = "[email protected]"

####################################################
# Optional settings.
# Here are some recommended configuration variables.
####################################################

BARE_APPEND = "~/.tmda/lists/whitelist"
CONFIRM_APPEND = "~/.tmda/lists/confirmed"
LOGFILE_INCOMING = "~/.tmda/logs/incoming.log"
LOGFILE_OUTGOING = "~/.tmda/logs/outgoing.log"
LOGFILE_DEBUG = "~/.tmda/logs/debug.log"

TEMPLATE_DIR = "~/.tmda/templates"
#CONFIRM_ADDRESS = "[email protected]"
#HOSTNAME = "hplb.hpl.hp.com"


.mailfilter is:

######### Setup the useful Globals
SHELL "/usr/bin/ksh"
LOGFILE "${HOME}/Maildir/log"
MDFILE=`/bin/date +%m-%y`
DEFAULT="${HOME}/Maildir"
PATH="$PATH:/opt/maildrop/bin"

`test -f vacation.msg`
if ($RETURNCODE == 0)
{
         if (!/^Precedence: (bulk|list|junk)/:h)
         {
                if (!/X-HPL-MailScanner-SpamCheck: spam/:h )
                {
                         cc "| mailbot -d vacation.db -D 5 -t 
vacation.msg -A 'From: [email protected]' /usr/sbin/sendmail -t 
-f ''"
                }
         }
}

############  Catch all rule
#to $DEFAULT
#
# TMDA
#
EXTENSION="$1"
#HOST="hplb.hpl.hp.com"
HOST="$2"
SENDER="$3"
if ( "$EXTENSION" ne "" )
{
     DELIMITER="+"
}
RECIPIENT="$LOGNAME$DELIMITER$EXTENSION@$HOST"
# "to" will deliver to message to tmda-filter.  It will exit with the
# exit code of the program.
to "| /opt/tmda-1.1.9/bin/tmda-filter"

Sendmail is v8.13.4, TMDA installed from the .tgz


-- 
John Hawkes-Reed
RIT. HP Labs Bristol
+44 (0)117 312-8787