[Exim] BSMTP To: header rewritten, normal incoming mail does…

Hello all...
[and first: a Big Thanks to Philip for a _great_ piece of work!]

I am trying to track down a behavior difference in exim's handling
of a To: address in the rfc822 headers when the message is received
via standard SMTP and when the message is received via batched SMTP.
Exim 3.36....

An example follows:
#!...as a trusted user...
# /sbin/exim -oMr processed -bS
MAIL FROM: <joe@???>
RCPT TO: <someuser@???>
DATA
From: <joe@???>
To: JOE
Subject: atest

testing
.
QUIT

This results in a message arriving at destination with a
header:
To: JOE@???

Where 'exim.test.com' is the name of the mail transport machine.

So far so good... [and I'd like to understand how to stop it
from doing that, but that's another question...]

*However* if I telnet to port 25 on the machine running exim
and go thru the same SMTP dialog, the message is delivered at
destination with a header:
To: JOE

Now the $10 question is where did I goof to make exim behave
differently in these two cases? Any pointers appreciated.
Ideally, I'd like to get control of when and if this rewrite happens,
BOTH for causing it to happen from the normal SMTP port, but
also to disable it on BSMTP.

Regards.
--
David North