[exim] Space being converted to equals in email address

I have been alerted to a situation where a space is converted to an equals in a quoted email address.

I send a test message to "/sRE: Testing on sending a doc 3.01632990099"@???'

This is logged as:

2017-09-07 12:27:04 1dpvtg-0005rG-Pn <= neil.youngman@??? H=hr-ops-1.wirefast.com (test.wirefast.net) [192.168.221.65] P=esmtps X=TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256 CV=no S=1200 for "/sRE: Testing on sending a doc=3./a01181368365897\n./cRR0Y3YAVKZ3E7HJ9-001.01632990099"@???

In the message itself it is configured to add Envelope-To, which comes out as:

Envelope-to: "/sRE: Testing on sending a doc=3.01632990099"@???

There is nothing in the rewrite rules which would explain that substitution.

THE Exim version is 4.86. I don;t have an instance of 4.89 handy to test against, but I will run one up soon.

Does anyone have any idea what could cause this?

My test script is:

=======8<=======

#!/bin/bash

SWAKS=/usr/bin/swaks
SENDER=neil.youngman@???
DEST='"/sRE: Testing on sending a doc 3.01632990099"@???'
SUBJECT="Test from Neil"
SERVER=mail6
HOST=test.wirefast.net

sent=0
failed=0

for n in {1..1}
do
$SWAKS -tls --from="$SENDER" --to="$DEST" --server "$SERVER" --ehlo "$HOST" --suppress-data --data - <<EOF
From: <$SENDER>
To: <$DEST>
Date: $(date --rfc-822)
Subject: $SUBJECT
Content-Type: text/plain
MIME-Version: 1.0

This is a test message from Neil

Please ignore this message

Thank You

EOF
  result=$?
  if [ $result -eq 0 ]
  then
    ((sent++))
  else
    ((failed++))
  fi
done

echo $sent messages sent and $failed failures

=======8<=======

Neil Youngman